Use a quarter round trim strip to form the corner, where you have a
radius on the outside, and a square corner inside, which you can trim
or fillet. For other than 90 degree corners, trim the strip on a
table saw. Use thin plywood and build up the edges to meet the trim
strip depth and accept screws holding it together.

Arrange to remove the plywood mold plug from the inside by covering
the outside with waxed paper. After you have built up sufficient
thickness of glass, drill out the screw heads and remove the screws to
remove the mold panels. Fill the screw holes.

I considered just glassing over the old canvas, for that slouch hatted
look.

Terry K