I know it's probably better to just have 1 question rather than 4 or 5 b/c they won't all get answered or might be too much for folks to read, but I'll see how this works.
I've got an older Delta Shopmaster (about 2002) that comes with a blade insert that leaves a gap of about 3/4 inches around the blade. I've checked online and can't find any zero-clearance ones (well near zero). I feel uncomfortable cutting wood with that much clearance around the blade and already had wood sucked into the hole and explode and other nasty surprises.
The newer Shopmaster ones I saw have oval ends but mine is rectangular in shape.
I made 3 zero clearance myself but they rise slightly above the table b/c of the design of the insert. This in itself poses safety problems.
1. You should plane the insert material down to the correct thickness or find a material that is already the correct thickness.
Second issue. It would be nice if the plate includes a splitter. The splitter that comes attached to the plastic guard is a couple inches too far back to be really safe. Also, when you remove the guard to do raised panels and stuff, the splitter gets removed as well. 2. There are after market splitters that you might want to check into. I believe you may find one of them at Rockler.
Third issue. If I build a sled, do I still need a splitter or does the sled itself act as one? I've seen about 20 sleds people made and only 1 had a splitter. IOW, are they safe without splitters? 3. Sleds are safe without splitters, but you could still use one with a sled and be even safer.
Fourth issue. This involves the plastic blade guard. I read a story from a woodworker who was badly injured b/c he removed it and cut a large sheet of plywood. The plywood somehow got kicked back at him, injuring him badly in the groin. I guess the lesson I take from this is to leave the guard on unless you can't (like when doing raised panels) 4. The blade guard was put there for safety reasons. If you choose to remove it you are just asking for trouble. There are times when it has to be removed, but it should always be replaced when the operation is completed.
Fifth issue (I couldn't change topic title from 4 to 5):
I've had the plastic guard down while cutting and have had wood fly back thru the narrow guard and lodge into my finger. It hurt but healed. I started wearing leather gloves but found the danger there is that it makes your fingers more clumsy. 5. Gloves are not recommended, but there is a set of gloves with some of the fingers removed so that half your finger is still open. I got mine HERE