Stretcher Bars and Strainers

Hi All, I run a picture framing business and Im wondering if anybody on here as any tips for building large stretcher bars or strainers for artists canvases. they have to be strong and square not bend out of shape when a canvas or linen is stretched on them. Im currently making them from scratch and using crossbars with pocket screws and standard 45 degree joints on the corners held with glue and biscuits. Any advice on working big? 6 foot x 6 foot + thanks D