In the old days?... There was a Sorghum contact cement that was popular. It would go on with a credit card. We had a water based one here that worked well. After many years it would stop sticking and need to be reglued. Epoxies are good. I have a feeling that the foaming type of polyurethane would be the lightest, and maybe even stronger because it expands a bit to fill up the small gaps. It is put on with a credit card so the amount would be the same as epoxy. I have heard that it can slip when clamping or bagging. I was going to use a plywood top piece, and about 2" foam rubber sandwich with weights. Some of my cores have no bucks. Can't really say that I tried it, have enough wings for a few years, and my Gorilla glue dried up on me.. Lots of guys swear by it though.