How about a very small inflatable dinghy? Like a kids pool toy. Just big enough for all your gear but small enough to deflate and bungee to your float.
All gear would be secure for travel, you could partially deflate it to ease access when you're ready and you could reinflate the raft with the reserve you surface with.
The dinghy idea is where I was going with the UFO although that is also the float itself. I could look for a bit larger one to hold the tanks. Of course, the larger the float, the more drag it has while I'm towing it underwater. Deflated and bungeed, it would create even more drag in the current underwater than floating on the surface. So it's something I'd want to leave inflated.