When I got my RF30 clone more than 40 years ago, it came with a clamp set with 3/8" studs. The table slots are less than 5/8" but were too large for the 1/2" tee nuts that came with the set. One of my first milling jobs was making some tee nuts to fit the 15mm slots. I made a half dozen for 3/8" studs and while I was at it, another half dozen for 1/2" studs. They are fairly simple to make, I started with 6" bars of 1" x 3/4" stock and milled the sides to .580" and the height to .680" and then drilled and tapped for 3/8" and 1/2" every inch. They were then cut apart on the band saw. They have held up well over the forty years.
I got a 1/2" clamp set with my Tormach but I prefer using the 3/8" set. It hasn't ever let me down and it isn't as clunky. I also have 3/16" tee nuts in 1/2" and 7/16" size for other slotted tables. For short clamping situations, I will use 3/8" bolts. 3/8" all thread also works for custom length studs. It doesn't hold up as well as the hardened studs but it gets the job done and is low cost.