Sv are you a machinist? Could you take one and make drawings get the tolerances etc. Or start making then? There probably isn't a large market these days as very few are building from aluminum rectangular tube. Here are some pictures. By putting a drill in the bearings and using feeler gauges to measure the distance from the edge of the bit to the edge of the lip that would contact the edge of the aluminum tube I get 0.143 inches. Take 1/8 in from that (tube wall thickness) and it seems the clearance is 0.018 in or just over one sixty fourths inch.
You would have to do the math depending on bearing type used to get the center drill position. Here are some pics of my drill gauge from Jake.