Hi Joe --
I've attached a couple of photos that show the mount for the Colt on my Zenbot 1216. The idea would be to do something similar but sized for the Rotozip.
You didn't say what model Rotozip you got, and I understand the new ones are now Bosch Rotozip, with specs very similar to the Colt. Does it have a 'handle' like the earlier ones? If so can it be removed? Otherwise cutting the hoops can't be a simple circle.
I've seen a variety of non-factory spindle mounts and there are other ways to do it. Hopefully, someone can respond to you who has already done this one specifically. If not I hope this is of some help.
Worst case, you should be able to order the mount for your machine from Zenbotcnc.com for about $30. If you cut it yourself you will probably need a long reach end mill to cut the 3/4" HDPE profiles, and if you don't have the plastic on hand you may have to buy a bigger chunk than you actually need. All said, it may be worth it to order if from Shaun/Zenbot.