Yes, I also looked longingly at the tool batteries as well. A 3d printer could make a holder/connector and there are plenty of demo boards around for switching powersupplies to make 12V.

But I decided to go with three K2's. So far I've never used the third K2.

They were already tested and in the physical and electrical form the glider expects.
LiFePo seemed more forgiving than LiIon.
I'd rather not divert the hobby from flying to building battery systems.
It seemed a thing that would be fun and easy to get close, but tricky to get right.

If the K2's didn't work so well, it might be a different story.