Be really careful when cusotm molding Spec Ops liners. All of mine have been a thermo-plastic other than kydex, and have been far less forgiving than kydex. I would recommend hot water or a hair drier rather than a heating gun, and make minor adjustments at a time. If it starts to curl on its own it's getting too hot. Slow and patient is the best way to happiness with these.
Another consideration is the number of knives that you have that may fit into a single Spec Ops sheath. You may have several knives that will fit into the generic sleeve, but you could limit yourself if you custom fit the sleeve to a particular knife. I have two Spec Ops small sheaths that I can use with about 7 knives. Just keep good tension on the bottom most screw to keep the edge away from the rivet and you should be fine. A little movement in the sheath is a small price to pay for versatility.
Another option would be to commission one of the kydex guys to make a liner for you. Should be cheaper than a kydex sheath, and a perfect fit for your knife. I've done that with several of my Spec Ops, after ruining the original liners and discovering the advice I wrote above.
good luck!