How to install these seat posts?

[SkaterRace]

I bought the 7x7 seat bases that are used for the pin post seats. They are exactly what I want however the boat I am installing them into is an older one with a fibreglass deck and I assume foam below deck. These post bases go down about an inch or so but that is fine if I seal the top lip however they are also bottomless and water would just pour down in if I left them as is.
My question is how would I install them without putting on a block above deck? They are already higher than I'd like but fine for how often we need those spare seats

 

[dingbat]

You need a backing plate under the deck to support the load.
Scrap or cut away the foam in that area to expose deck. The 1/4" thick aluminum plate would do the trick. Put a bead of 4200 on the perimeter of the plate and through bolt with stainless hardware and locking nuts.

Can't see that much water getting in the hole, but if worst comes to worst, don't see why you couldn't install some sort of a plug to stop the water from getting in

 

[SkaterRace]

You need a backing plate under the deck to support the load.
Scrap or cut away the foam in that area to expose deck. The 1/4" thick aluminum plate would do the trick. Put a bead of 4200 on the perimeter of the plate and through bolt with stainless hardware and locking nuts.

Can't see that much water getting in the hole, but if worst comes to worst, don't see why you couldn't install some sort of a plug to stop the water from getting in

Thanks and for water getting in the hole, these seats won't be in place 90% of the time which leaves a 3/4 inch hole into the foam. I thought maybe a plug but it would have to somehow be 1/4 inch below the bottom of the base plate which would mean building a box below it.