"Bob" wrote:

I'm building a corner cabinet that has a face frame and plywood
sides.
I'm having a problem figuring out how to join the two plywood sides
at
the back corner. I can't picture how to clamp the sides together
since
they meet at a 90 degree angle.

The cabinet will have shelves so I want the inside of the cabinet to
be as clean as possible.

A combination of cabinet and boat building if you want a sharp corner.

If truncated corners are acceptable, then Swing's approach.

This assumes the sides are at least 1/2 ply. Could be 3/8 but more
trouble cutting smaller rabbets)

Cut a 1/4 rabbet in each ply piece.

Interlock rabbets together to insure a straight edge, then clamp short
blocks on each end along with a diagonal braces at each end, clamped
in place to insure 90 degree corner.

Get some 4" wide, fiberglass tape and some epoxy.

Flip unit into position outside corner up, to work on outside corner,
then cut 3-4 pieces of tape about 4" long and epoxy them equally
spaced on outside corner.

When cured, remove clamps that are in the way and epoxy a full length
piece of fiberglass tape to secure the entire length of corner.

When cured, flip over so inside corner is up, then glass the entire
inside corner with another piece of fiberglass tape.

When cured, sand lightly to remove any drips.

SFWIW, have built battery boxes for a boat that hold 300 lbs of
batteries with no problems using this approach.

Have fun.

Lew