Heh, this design is where I started! Funny to see it come full-circle (again).
For doing a one-off for yourself, this type of design is doable because I made some like this. However, it did require quite a bit of tweaking and recentering of the joystick to get good results. Because the Happ joysticks are so diagonal-sensitive, precise placement of the restrictors is critical. This is how I ended up having to go with a mounting plate so the joystick would be centered through the use of it's mounting holes with respect to the restrictor plates. I hated the idea of having to add a mounting plate, but it was the only good way I could make sure the customer always had a properly centered joystick and still make it simple.
For me to produce & support a sellable product, it needed to have repeatable and predictable results. The "drop-in" style restrictor was too flakey and relied too heavily on customer installation (exact centering of the joystick hole) for good results. My take on it is when people buy something, they expect it to work. If you build it yourself, you expect to tweak & experiment around a bit to get it to work.