I don’t see how the telescopic portion is useful without a lens on the front. I’m looking at the “Review: TrustFire Z1 XML-T6 ”1600 Lumen” Zoomable (a.k.a. Z5)“:Review: TrustFire Z1 XML-T6 "1600 Lumen" Zoomable (a.k.a. Z5) thread for reference here. If I was going to make the flashlight into a mule, I would remove the telescopic portion and create a new cap. It looks to me that the inner portion of the head is about 30mm, so I would try to find a bottlecap or copper pipe cap or other type of cap with about the right size, then cut a hole for my window to fit into. I would silicone the window in place to seal it, then force another piece of pipe behind it, then slip that onto the flashlight and silicone that in place.
Something like this:
