Here’s a small trolley I built a few years ago for my small homemade gantry. The gantry uses a 2” square tube (1/4” wall) for the cross beam. The side plates are cut from ¼” plate, the rollers are 6205- 2RS bearings, the “axles” are made from 1” stock 2-1/8” long w/both ends turned down to 25mm for the bearings with 9/16” between, then drilled and tapped both ends for bolts. Holes drilled in the plates for a load support thimble. A ½” nut was welded to one side for a T bolt to lock the trolley in position. Drilled the end of the thimble bolt for a hairpin cotter pin, for safety. The plates are 8” wide and 7” top to bottom, but could be any size, I just used what I had on hand.
The gantry was a little narrow for some applications so it was extended with 6"X6"X12" I beam pieces. Load tested to 700 lbs.