The stock insert that comes with the Jet CS (CTS?) is metal and has a wide opening capable of handling a dado stack. Today I took it out to make some zero clearance inserts out of 1/2" finnply. A couple of comments and questions.
First, the factory insert doesn't exactly fit snugly into the opening. It's a little too short and narrow, although a couple of little nipples on one side sort of guide it into place I guess. I ended up just template routing it as it was, but I plan to experiment more in the near future. What I'm thinking is that I'll flatten the nipples using some abrasive paper on a surfacing plate, and I'll take off the seam as well (all the way around). At that point I'll add a couple layers of celophane tape around the edge to make it just slightly too large, and template rout it again. Now I'll have an insert just barely oversized. This I'll take to the surfacing plate to get it just right, and I'll keep this for my new "template". (I'll probably make up a few spare inserts at that time too).
I've noticed too that the inserts don't seem to want to sit flat on the supports either. To level the one I made today I used ground down screws countersunk into the insert itself. This seemed to work pretty well, but I thinking that next time I'll fix the problem itself, which appears to be supports which aren't coplanar and/or on a parellel plane to the TS surface. With screws as leveling feet it feels like the back of the insert can easily dip. Maybe I'm just being anal.
IAE, what I might do is file/permanently shim the inserts such that they allow for an insert of 31/64" to sit flush with the surface of the table. That way I need not worry about leveling screws or rabbeting the edges in the future.
JP
************* No tolerance.