The only way I could get them to fit properly was to remove the switch, which I wanted as added pocket protection. The unprotected version may be shorter, but I like to err on the side of caution
They would squeeze in with a greatly shortened spring, but the positive terminal always seemed quite sharp, inserting the battery damaged the plastic around the battery.
I didn't realise it, but the negative at the bottom of the battery is connected via a bare wire under the plastic to the top of the battery, hence the short at the +terminal after the plastic was damaged.
The torch mod works very well and avoids this issue, the only difficult part was fixing the pcb for the battery mount, which proved a bit tricky, especially since I was worried about it touching wall of the torch, but wrapping around the pcb with tape solved the problem.
It doesn't look great, and I have gotten some peculiar looks, a friend thinks it looks a bit like a hash pipe

, but i'm happy with the 3.7-4.2 voltage range, and the battery life is a big improvement over the regular DS901 battery.
After reading some of the advice on this forum I have ordered a DPV9 PCC kit with a couple of batteries that I will use with an RN4075 atty. These batteries seem quite good and I love the idea of the PCC. I also like the idea of the DPV9 carts, but for that kind of price I would want to refill them, and I read a few worrying reports about the filling burning.
Anyhow, thats enough of me rambling