The minimum bolt diameter is pretty much a function of what kind of loads you want to clamp down, home many bolts you want to use, and what kind of bolts you use. If a couple of grade 2 bolts will do it, ok. If not........
Mcmaster sells square head bolts in 1/4" size, these are ungraded, not sure if that means grade 2 or the grade 5 that is mentioned in the overall description.
I'd suggest making your own 'T' nuts or just chunks of bar stock with holes drilled and tapped in them. You could make your own table with some heavy bar stock, lighter bar stock plug welded on the back to keep the nuts from spinning, and the heavy stock welded to a frame with the gaps set for the bolt sizes you want.
If you do any sort of repetitive bolt and unbolt cycle, you will f> Hello,