I rip boards to make my supers and rabbet the joint and frame rests.
Waste wood from the rip gets cut into handles nailed on the long side of the
bodies and glued.. Nail from inside the box  into the handles and through the
handles into the box.  8-10, 4d box nails in each handle.
Also make some shims for your supers to turn a medium into a deep with a shim
made from scrap.
You could use longer nails and nail from the outside through the box and then
fold them across the grain-these wont ever come off.
Tom in CT