The grip mounted type would be nice, like they made for Glock Poly's. I am not a designer, but it would seem they could come up with something that would fit the PT series, XD series, etc. As they are almost the same size in the critical area's.
Actually, Laserlyte makes one that fits on the trigger guard of many guns. You have to select the right inserts to make it fit correctly, and clamp it on the guard at the front. I bought and used one when I was learning to shoot my PT111, and it worked pretty well and was steady.