I'd say the best bet is to go with something sleek, thin and plastic. Which rules out pretty much any
GW humans!

Instead, I'd look to Eldar, of both varieties. You'd have to remove a few gems and armour plates, but Dire Avengers would be a good start, and the new Harlequins might have something that could work as well. It would take a bit of conversion, of course, but I'd start with
DA as a base model. Maybe use a Servo-skull as a head.
Of course, that assumes you're wanting something close to the 'official' look. If you are willing to deviate a bit from that, then the possibilities are endless.
DE Wracks could make awesome experiment-gone-wrong style monsters, an Empire Flagellant in some kind of harness could be a captured Blank (counts-as Culexis). If you feel like being creative, you can do anything really!