The glass beads make the lowest density filler and are the lowest cost. White wheat flour from the kitchen is a pretty good filler for finishing putties. A fiberglass-epoxy butt joint of plywood can be as strong as the plywood itself. I tack such a box together with brass brads and then make the epoxy fillet joints. All joints must have the weave of the glass cloth filled smooth for maximum strength.
Many designs specify much more glass than needed.
Make up short specimens of your joints, cure them, and test them-in a vise, by standing or jumping on them, or by running your truck over them. If the joint holds and the material breaks, your joint is strong enough. The principal hazard of working with epoxy resins is from skin contact. The hardeners are the offenders. Wash thoroughly before eating, drinking, or going to the bathroom.