using piston stop
The commercial ones are not very well thought out. You need a jam nut on the rear of them to lock the plunger. As above it must be rigid this is the one I made for a SB in the 70's and (miracle) LOL still have it
It's not very important how long it is except that you don't want it too close to TDC. You want the piston to stop "down a ways"