There are a couple of threads dealing with Primo shoes on this forum. You might take a look at this thread: Dust collection without shoe...futile?
There are a couple of workable designs there. I included a Fusion 360 file that contains everything but the shoe itself for a zero-clearance, core support shoe in that thread. It gives you a starting point if you want to design your own shoe.
In shoe design there seems to be some competing issues/choices:
- How much X and Y travel do you want to give up?
- How much Z distance do you want to give up?
- How easy is it to attach the shoe?
- How easy is it to see the work while cutting
- Is the shoe attached to the router/Z axis or to the core?
A shoe attached to the router will likely work better in terms of dust collection but at the cost of additional mass and resistance on the Z axis. This resistance will impact cutting to some degree, and I believe Ryan recommends attaching the shoe to the core.
I’ve made two additional shoe designs since I made the post on the mentioned thread, and am still not happy with my shoe.