Using bolt cutters I snipped each of the shoe loops so it was only a single rod. I suppose I could have just snipped off the top of the loop and had 24 rods instead of 21, but the spacing would be very tight.
I then epoxied some short dowels (5/8" x 2 in. for 18mm motors, 1/2" x 1.5 in. for 13mm motors) on the end of each rod, so they wouldn't tear the motor tubes.
I made the top tier for 13mm, the other three tiers for 18mm.