I'm making one for car valve springs, just made out of scrap steel. It wont work with side valve engines but it will with OHV stuff.
You shouldn't need a valve spring compressor for work on industrial engines, on OHVs you can just push the collet off with your fingers, with sidevalves you can just use a screwdriver and a set on pliers. A proper compressor is pretty much needed if you've got split retainers in a flatty though, it's **** near impossible to get them in without one. OHV is about a million times easier for this.
You could modify the G clamp quite easily probably, just a bit of cutting and grinding really.
Thanks
Hayden