I've moved away from frames altogether for large works, unless formality of the exhibition space truly calls for framing. Even then, Exhibition Glass appears to come in 40"x60" sheets, so there is that limitation. I personally and for idiosyncratic reasons, eschew plexiglass.
We mount large prints to 1/2" Gatorfoam and laminate the print with a luster material. The appearance is quite good. I do note that thicker papers may separate some at the face edge, and this may obligate use of a cap extrusion to hide the very edge of the print. This separation has only been an issue with Photo Rag Satin, in our hands, but I'm sure other papers will separate at the edge. On the other hand, when the print is on, say. Canson Baryta, the edges look quite clean once mounted and laminated.
While this may offend, quite deeply, the archivists - these are inkjet prints and I'll be probably dead before they look bad. The party displaying the print will like have lost interest in the work by the time the print has degenerated. Your experience may vary.
Gatorfoam is available in 60" x 120" inch sheets, I'm pretty sure, but I've not made a canvas this large.
John Caldwell