Certainly it's possible to plan for that during building.  Just add a couple of straps like the hatch straps found on the chesapeakes and you should be good to go.

FrankP

i plan on adding alot of those looped webbing tie downs in fact im doig bow and stern perimiter lines [leaving the middle 5 feet or so without obstrution] ym more cocerened with tha additional weight  t[totalabout225 ] with me and my gear

Well, the Kaholo is supposed to have a max payload weight of 275, with a "paddler weight" of 50-250, so 225 should be fairly safe.

I've found the weight limits tend to be conservative as maximums, though with a lot of additional weight you can see a slight (depending upon the boat/board design) performance hit.

FrankP

im thinking of using some bungy netting hooked onto webbing loops in the shearclamps with 2 pieces of pvc pipe attached so the tank can lie between them and then just a strap over the top

PVC?!?  Seems a shame to go to all the work to make such a nice looking wooden work of art only to attach black plastic pipes.  I would suggest that you look into bending wood.  I was amazed how easy it was and the great results.  And that was with plywood.  I bent 6mm for the coaming of my SOF in a radius far tighter than you would need for a tank.  If you do not want to get into bending another option would be to glue together a couple pieces of scrap to give you some tickness, then simply cut the arch you need and epoxy that to the deck, couple holes for straps/netting and you will have a line up admiring your craftmanship.

pvc not a permanent attachment  only clipped onto web loops when carying a tank

greg27, I'd agree, making a "pretty" cradle wouldn't be too difficult.  Personally, I'm the kind of person who will make do with what is efficient and gets the job done so PVC works, but if you're going to the trouble of making such a nice platform, spending the extra 20 minutes to build a wooden cradle would look really nice.

FrankP

lots of time to consider.actualy im ore consened with protecting the board from getting dinged by the tank, maybe i'll attach the cradle to a mud flap or something