For my Passagemaker Dinghy, I initially used a couple of old mouse pads folded over and stuffed into the slot.  Upgraded that to a two strips of adhesive backed weather stripping glued back to back with a loop of line sandwiched between to use to pull it out.  Works well, and stows easily when not wanted.  I don't know that plugging the slot with something fitting flush to the bottom would be enough of an improvement in rowing speed (best I've been able to do with the PMD so far is 4 mph) to be worth the trouble.

Don't know whether parts for some sort of plug are supposed to be included in the Eastport Pram kit, but there was no mention of such for the Passagemaker.  Seems to me like I emailed CLC a question on that, which is probably where I got the foam suggestion.  Might be worth a call to CLC to ask about it.  They've always been friendly and helpful any time I've emailed or called with a question.

.....Michael