I'm anxious to see what others suggest, and I don't have much knowledge of the design parameters you're facing, but would installing a short piece of pvc pipe help?  I'm thinking of it as a below-deck sleeve with an inside diameter suited to your mast.  That would at least allow you to have a flat rather than curved top, and that would seem to open up more options for some sort of cap.  Keep us posted on what you decide.

Taking an idea from the flush hatch idea, perhaps rather than drilling a hole you could cut one out with a fine saber saw or japanese pull saw,  then fashion a gasket to match the width of the saw kerf  and a wood handle that extends just beyond the size of the hole.  This could then be pressed into place to fill and seal the hole and pulled out when you want to mount the mast.  A short string could tie to the handle and to a fastener of some sort on the deck to keep the piece from being lost.

Or you could venere whatever cover you fashion to match the deck. 

Part of the fun of homebuilding is that you can create solutions and change them if you are not happy.

Ed

I've always had good results in hatch-cutting with a variable-speed saber saw fitted with a fine-kerf plywood blade. But if you don't trust the power option, you might consider the bonsai saw that CLC sells:  http://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/booth-tools/bonsai-saw.html 

Another option: you could shape a wooden plug with carving, planing, filing and sanding, and maybe top it off with a decorative inlay or onlay, or maybe encapsulate a nice design printed on rice paper under the fiberglass and epoxy.

Keep thinking, and you'll come up with something special. The biggest satisfaction of boat-building is dreaming and problem-solving!

Good luck.

Jim

   I couldnt wait to sail my rig and went out with the bare hole, big mistake.  The rig has some ability to shift around and the mast working on the hole edges delaminated the fiberglass all around the hole.  Not to mention the compression damage to the thin wood deck.

I have a plan to make a wood plug/sleeve that is much stronger and is formed to the deck curve, but that is a custom job that is well down on the to do list.

Keep in mind whatever you do that the whole rig is just lashed together, and there is a suprising amount of movement possible.  That aluminum mast if rubbing up against the hole edge can do some quick damage.

I am interesed in seeing what your solution ends up being.

Dave

 How about a small compass!

  What's the diameter of the mast?

Would something like this serve to cover/repair the delamination damage and prevent it from happening again? 

http://www.rockler.com/fastcap-dually-double-sided-plastic-grommets

If not this specific one, than something similar with the correct diameter? Perhaps in conjunction with a PVC tube, as has been suggested earlier, fastened securely to both the deck and the hull?

Jim

   Perhaps it's just my twisted sense of humor, but I rather like the idea of a fitting on a 12' wooden kayak that reads "Gas".  But anyway, I'm currently building the same boat (WoodDuck 12 w/SailRig) and have been kicking around ideas for a reinforcement for that hole in the deck.  The Rockler grommets are too large, but Lee Valley sells a range of similar desk wire grommets in sizes 1" and up, in several colors.  My possible wooden solution involves a piece of durable hardwood, such as white oak, and a set of hole saws.  Basically, this would require boring an oversized hole in the deck, approximately 1/2" to 3/4" over your final size.  Then use another hole saw to cut out a "plug" that fits the hole in the deck from your piece of hardwood, and finally use a smaller hole saw to cut the final through hole for the mast after aligning it with the hole in the aka.  This would give a final fitting with a wall of about 1/4"-3/8" all around.  The "plug" would be epoxied into the deck, held in place with tape while the epoxy sets up, leaving the fitting a little proud of the deck camber to be filed and sanded flush afterwards.  As for a way to seal this off when not in use, I'm currently leaning towards fashioning a small rectangular hatch cover, laminated from thin ply or solid stock, which matches the deck camber, held down with a couple of straps and with gasket material undrneath.

   I haven't done the sailing option but I would think that a couple of layers of 3mm. Okoume epoxied under the deck running from gun'l to gun'l in the manner of a mast partner would help. I like the idea of using the piece of deck left over from cutting the hole and that would be an excellent place for a compass.

   Wow, excellent replies!   Thank you all, and keep 'em coming - you've suggested several things I hadn't thought of. (For those not familiar with the sail kit, the mast diameter for the 40 and 55 sq foot sails is 1.5 inches and you sleeve the sail down the mast to just above the AKA with a light downhaul on a cleat, I believe for the 70 foot rig the mast is 2 inches, and you attach a sail track to it). The compass Idea is nice, as is the slightly oversize 'hole sawed' fitting within a fitting - perhaps combine these two.  Peter, I will check out Rockler and Lee Valley, I had been looking mostly at marine suppliers, but you raise good points (I too smiled at the idea of a 'gas' cap, definitely funny once, but it might get old).   I will continue to ponder and watch this thread for other bits of brilliance. Based on the replies I see I'm not the only one who has stumbled at this, I suspect this thread may help others in the future, so I would think all ideas, specific product suggestions (I have not so far found a 'ready made' solution for this), and insights into repurposing something would be useful. 

-Ralph

   Okay, so here it is in-progress. I cut the prototype from some scrap cedar, the final is maple. I'll post another pic when I get it installed and finished.