i checked with lowes and HD looking for a bilco style door, but ours seems to be ALOT smaller than normal. the opening is roughly 4x7

i want to constuct a door for temp use (a year or 2) untill we have the funds to have a custom one made.

my plan is to start with a 2x4 frame, then plywood, then 5/4x6 decking boards. my question is what can i use to make a good seal between the wood and the concrete?