X3 Don't spray. Also don't use undercoating, if you don't have an absolute seal moisture can penetrate and it won't be able to dry out, next thing you'll be replacing your floors.
Check Home Depot, in Canada they have a product called Roxul rock wool. The fire barrier version can be totally saturated with water. It can be taken out and dried if wet and retains it's shape and R-value.
I wouldn't worry about trying to get the coroplast sealed up totally, if you do get any water in the belly pan you want it to have some way to get out.