Blacksmoke

I'm trying to see if my 18 year old raised planters with palms is ok for my young banana plants.

I dug a hole 12" x12" although it was in an existing small palm root so to get to 12" deep I had to move over another 12" so it was 12" deep and partially 24"" in width.

I had filled this hole with water twice in the preceding days and yesterday filled it again and did timed drainage (MPI) which seemed to go well at 15-30 min. Per inch.

This am there was still 1-2" of water in the bottom.

Should I dig and fill another 12"-20" deeper with pumice or the like?
Add a 24" wide wood box 12" high over it to raise the root ball or just keep it in a huge pot?

My goal is to get them to fruit.

Any tips?

Thanks

Scott