Well,
I used Epoxy for years, think I had to for strength. Then a freind that built and raced quickies for ever told me I was just wasting money and He for years has used plain old Carpenters glue

So I tried it, Works great and it's Cheap! Also I like the fact you have plenty of working time to get things positioned

What I do is cut it with about 30% water so it will spread out thinner. Then spread it out over the Foam, Not the balsa, as thin a layer as will still cover the panel.
I found if you spread it over the Balsa it has time to start warping before you get the weight on it.

Summer time it's ready over night, Cold and humid weather will slow the cure time. If you pull the weight off and it feels damp and cool, it's not dry yet. Usually you can feel it through the balsa

If you use water based glue make sure you have no dings or voids in the foam panel, the glue will be to thick in that spot and take longer to dry, Then if you pull the weight off before all the glue is dry the wet spots will cause the balsa to bubble a little in those spots. learned that the hard way on a not so perfect panel I cut.

Good luck.