3/4″ strainers are made from a single layer of Baltic birch plywood and are available up to 36″. (Note that 1/4″ [6mm] staples or an air nailer must be used on the birch to ensure the staples pull in tightly).
Larger frames are made to be 1-1/2″ by adding an additional layer of fir plywood laminated to the back. This both adds greater stability and is also easier to staple into than the birch. (Staples longer than 1/4″ [6mm] may be used on the fir.)
Round and oval frames 40″ and larger have integrated bracing and those over 48″ are also made in sections that bolt together for easier manufacturing and shipping.