I have always expoxied backing pads in position. I think if you used sealant it would probably be as good, but I am not sure there is any benefit.
Sealant is used as a non-permanent bedding material – e.g. under fittings – I don’t think that applies here.
I would say 3/8″ marine ply was about right and I would try to put as bigger peice in as I could. Perhaps that’s overkill – but I like to have confidence in my boat.
Blutak describes the plywood he used above.