A bolt stretcher would have required much heavier wood bars. In our experience there will be no need for future stretching - this poly/cotton inkjet material is quite elastic. This entire stretcher frame only weights a couple of pounds.
In our shop we use various mechanical hardware when needed for things like heavy rugs. I prefer using angled turnbuckles to cross bolts because I don't have to drill the bars. Do you remember a product called "Stretcher Wedgers", I still have few and wish they were still available - great for medium size, heavy pieces.