A quick and simple idea would be to take preexisting morphs stat blocks, like the Fury and Sylph, and then make a few cosmetic adjustments, maybe exotic physiology. With claws and horns as ware included in the new 'base' morphs you could set it up pretty easily. That's the easy way. The harder way is to stat it out yourself with bonuses where it seems appropriate and hope you balance it reasonably well. Make sure to compare the end result with other finished morphs to make sure they seem right. I think the easy way is probably the best solution here, as it should work well and is fairly simple. If you do have it be a modification in game then it also means it's easier to get 'their' morph on other habs by getting the base and modding it. That may take away some of the lustre from having a custom or relatively unique morph though. Up to you and what you think would work best for your players.