There is also a degree of common sense, I'd love to try more custom figures, I have a design for a Shad Space Trooper that requires Muton limbs, Kraken torso and a Shadow head but there's just no way I'd hack up a Kraken. That said, I'm sore tempted to take the limbs from my Night Patrol fig for Black Wolf but then I'm not into gen. 1. However I have held back as I know he's a rare one, I ummed and ahhed over cutting up Red Wolf until I won one cheap on ebay. I'd certainly never customise a Minesweeper of Gaucho fig.
As for Q Force customs, I'm working on a helicoptor at the minute. I won a Chap Mei Wild Arctic chopper on eBay for 50p, and it's the only one I've found that comfortably fits the TTBP landing pad.
