I have an artist who wishes a stretched canvas framed with a fillet/slip. The customer doesnt want his painting cropped the total of 12/15mm though, he wishes it to be 5mm at most. It is quite square. It's about 70cmx90cm. Best method to proceed?
I think your best option is to build a set of strainers closer to the actual size of the frame and tack it to the painting. The strainers don't have to be as thick as you would use to mount a canvas. They are just for filling the space.
Just to be clear about this, the customer wants a frame with a fillet/slip in the opening, right? As others have described, you could fabricate spacers to fit.
But if minimizing the coverage of edges on the painting is the plan, you could show the whole surface if you mount the stretched canvas into a floater frame, and put the visible frame & fillet/slip assembly on top of that.