This would be very useful to me. There is definitely a gap in the market for mounting plates and not just for T-Stiks.
I like to experiment with different types of joystick and it's annoying that there are two different bolt pattern standards out there.
I think about $10 would be a fair price.
This reminds me of an ancient thread I started over a year ago.
http://www.arcadecontrols.org/yabbse/index.php?board=1;action=display;threadid=7194
Would you consider manufacturing your plates to this design so that the T-Stiks could be mounted on the underside of the control panel?
Very interesting read!
I've not mounted my T-Stick Pluses yet, but the shaft length is the same as the Happ with the wood control panel spacer.
I've got 2 Ultimarc T-Stick Plus Joys, 2 Happ Super Joys & 2 Happ Ultimate Joys. Other than the Happ adapters for wood or metal Control Panels, I see no height difference in between the Happ (with wood adapter) and the T-Stick Plus.

According to the Ultimarc site, control panels thicker than 1/2 inch need to be routed out if the T-Stick is being mounted underneath, in order to maintain the stick length above the panel.
On my original NBA Jam Cab, the Happ Joys were mounted from the bottom with no routing of any sort. This had a combo of wood & metal that equalled 3/4" in height. How high would you want the sticks to be, above the control panel?
I can make either design & thank you for bringing this to light!
I need to make a couple of experimental pieces before I settle in on the proper design. I'm fully open for suggestions and want to contribute to the community.
Scott