After having to bust a double pane, tempered, safety window with a mag last night, I realized this may come in handy.
There are several premade options that replace the tail cap, but I figured $30 for something I may or may not ever use again was silly.
This took 10 minutes and was free from stuff laying around.
take the tail cap off, remove the spring and spare bulb (unless its an LED model, then its just the spring)
drill a hole to fit whatever size bolt you're using.
Here's all of the hardware - a bolt, a couple of washers and a nut.
I used a 3/8"x1" grade 8 bolt - screw a nut all the way down it so that it will fix the threads for you after grinding - by removing the nut, it will usually dress the threads for you.
A couple of minutes with a bench grinder put a conical point on it - it doesnt need to be a sharp tapered stabbing point - wider is stronger, you're just trying to focus all the inertia into one area
put the bolt through the cap - I used a pretty big washer on both sides to help distribute the impact, the cap itself is pretty thin.
thread on your nut- I used a nylock I had laying around because it was thinner and wouldn't come loose. and you now have a glass/head breaker