Acra Glass or it's newer filled variety will work. I get 2 part expoxy at hobby stores/Home Depot/Lowe's and used fumed silica to thicken when doing boat repair.

If you can find a copy of Crossed Chisels/David Trevellion's book about shotgun repair it shows this same process of grip repair.

The best plywood to use is the hobby shop stuff for RC airplanes. It is burch and 5 or more plys for a 1/4 inch. Usually it will take about 3/8 to make up the proper thickness for a trigger guard width.

W. E. Boyd