I don't doubt that you could make one yourself from scraps of ply and hardwood. Whether or not you should do that though...? :duno:
Essentially, the 'ring' needs to have a diameter greater than the the diameter of your largest cutter block. Therefore, giving you that main point of contact and enough room either side to 'lead-on to' the cut without plunging straight in to the rotating cutter.
Just found this one below, which is a pretty good example. I think it's made by SUVA and is supplied as an after-market accessory.
More info, here.
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It goes without saying that guarding is also important! :shock: Some kind of extraction port behind the guard would also be a good idea. In fact, you may be able to construct something that simply fixes to your existing fence, as opposed to something free-standing, as pictured above. Obviously, that may have some restriction and you curves and radii you could shape without the main fence getting in the way.