Hi!
Sure, it'll fit! You may even add a dual strobe 'cause the plus in positive buoyancy will compensate for it ;-)
I'm not an ewa-marine user anymore. That was over 25 years ago, when I was young and stupid (now I'm old and dumb ;-) and the OM-1 was launched.
I learned about the limits of the plasic bags quickly. I wouldn't take it beyond surface if I hadn't to. I just thought that the benefit in weight might outrun the other drawbacks while shooting above surface.
I just remember the few shots I tried to do with my 10kg rig. Nothing to take images of a speedy surfer zapping by. I thought more of a sturdy housing that's protecting the camera while being lightweight too. The bare bruder housing could do the job. Just as many controls as you want. Maybe he can do it even lighter if it doesn't need to stand 80m.
Another option would be a remote control to trigger the shutter. No zoom, no menues, no fancy stuff. Saves another hole in the housing. Leaves 2 o-rings (port and back) which could easily made double rings to add redundancy.
Helge ;-)=)