If you're just trying to tighten up the sheath, an oven is going to get too hot too quick to do this, IMO. On a finished sheath you risk the rivets pulling through and then you're going to be sheathless. I think the best route for this is a heat gun with a diffuser and being REALLY conservative with how much heat you put into the material. I'm not a veteran in Kydex but I've messed up enough to know what not to do.