Another option, weld a full-width tube under the frame tubes, instead of separate little tubes through 'em. Then cut out the middle bit, leaving two tubes on't rails fer bolts. Or leave intact if using studding.
Going across like where the tube is going across in your pic.
Maybe could notch out the back ends of the rails, nr the back light, and weld on a tube right across there? Can't tell if there's enough room with light gubbins though.
Most steel stock holders will keep some small diameter tube in stock. Can't remember sizes off the top of my head, but something like 1/2" OD 14 gauge can be sawn up to make crush tubes for 8mm bolts, and 5/8" OD 10 gauge is probably what works for 10mm bolts.