I would kit-bash something from another kit. Like a random splinter rifle mixed with an extra sail, or something along those lines.
The actual Wracks come with a few spare parts, like 3 extra masks and 4 pairs of arms (plus special weapons).
I did my conversions with the Fantasy Crypt Gouls (cant recall exact name). 15 "real" wracks made nearly 20 converted wracks (I think I was short a few bits that I stole from other kits).
Good luck buddy!