PF I'll suggest simply slapping a layer (or two?) of common duct tape over each hole. Or maybe packing tape, the clear plastic kind. If your centerboard trunk  sides are smooth, the tape'll stick well enough to prevent water intrusion (which I think will happen when you put anything into your rowing!) yet be easily removed when the time comes to put your centerboard into place with its pin. If there's any residual adhesive left behind usually some mineral spirits or denatured alcohol will remove it pretty easily.

Or maybe look for a pair of suitably sized rubber stoppers in your nearest hardware store? Something tapered so you can jam 'em in but still have something to grab onto when you need to.

   It is pretty low on the trunk so I'd think it needs to be capped.  Otherwise, at best, you'll be mopping out trickles of water and at worst have a real leak.  Look at the Construction Gallery on this site.  You can see the covers that are used for the pin.

I see no reason you couldn't assemble the hole covers dry, with maybe a cork or rubber gasket instead of epoxy fillets.  Then when you get the sailing kit, you just have to use a razor knife to cut through the varnish coat, pop off the covers and you are ready to assemble after sanding a bit of varnish out of the way.

   Just remember to coat all the cut surfaces in epoxy first so moisture doesn't find it's way into the plywood.

If you are rowing and the centerboard slot is open, you'll have some turbulence from the open slot and some water sloshing around in the trunk. Maybe you should consider covering the entire slot with Gorilla tape to keep that from happening until you do finish the sailing kit. Gorilla tape is remarkably strong and quite water resistant. I use it to patch skin-on-frame kayaks when necessary and one patch can last for years!  

   So you have centerboard pin hole in the trunk.? 

Just put a pin , aka bolt, washers and nut , in the hole with some  4200 caulk.  Then make a plug for the top of the trunk.  I have a T-plug in the top of  my dagger board hole when the boat is rowed or under power.  Water splashes up.

I wouldn't worry about the drag from the CB opening in the hull. It isn't a racing boat. .