My idea was to support the wheels from horizontal 2x4’s since they’re rather inexpensive, but I needed a way to attach the wheels to the 2x4.  I used some 1/8” aluminum angle iron that I got at the local home center (I had tried some 1/16” aluminum angle iron first but it was too flimsy - definitely needed the 1/8”).  I cut three 8.5” lengths (approx), drilling a hole at one end to stick the axle from the wheel through and four other smaller holes, two per side, to put screws through (galvanized 1-1/4” drywall screws) to attach to the 2x4.

Note that in the above pic the piece of aluminum on the bottom where the 2x4’s are connected is to shim the board to make the wheel level since it was twisting to the left.

BTW, this project was made much easier by having a drill press.  While a lot of this stuff could be done by hand, the drill press made everything much more accurate and quick