Use Cross Members similar to these so End Bracket corners are flush with each other and not overlapping. 

This is a 4 ft. x 8 ft x 2 in thick sheet of Styrofoam used as a set backdrop for a children's theatre play.

Wooden slats were glued on the back providing

corners, like on stretcher frames, to attach the End Brackets.

Using the Windmill Easel for set production saves time, and energy. You don't have to get on a ladder or bend over to paint the set.