I purchased one of the first Seatool Housings and have used it for two seasons.
It is very compact, nearly as compact as a compact camera's housing and much smaller that my Ikelite housing for the D-200.
Originally I use only a single Z-240 strobe and the included ball mount on the housing. No tray or arm at all. Just a hand strap. I doubt that their is a smaller DSLR setup anywhere. Later I added a single arm and second strobe plus a focusing light ball. Still very compact.
I use INON Z-240 strobes and fibre-optic connections--essentially foolproof. The combination allows more than 300 shots on the batteries.
The housing works flawlessly with all controls available. Other mfg's ports can work.