https://www.amazon.com/dp/B0BGQB485X?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
You can get a set of 4 for $27- It needs a modified rear caster wheel mount (which is on here) I would wait for the designer to reply.
@NWCC He can set you strait - there is another thread on this