Ok...before anyone gets too critical about design or depth limits on this thing, let me say that this took about 2 hours to put together. It uses only standard parts from the Sport Kiss. The most difficult part was rotating one of the canister/pod connections 90 degrees so that everything would easily slide into the case.
When the unit is not in use, the canister slides all the way down so that the pods and electronics are protected. All that sticks out the top is the BOV. This allows for a couple of open circuit breaths right away, while you undo a single clip to allow the unit to slide up into diving position, which also clears room for the counterlungs.
In the video, I have a 13 cubic foot tank and a 6 cubic foot tank. I have since changed to use only 6cf tanks, and the unit is very small. You could also plumb in offboard gas easily by running a hose from the 1st stages.
I have had it to 110 feet with this configuration, and it worked perfectly. It doesn't breathe that well if you are not in a horizontal position...but so what?
You could either side-mount the unit, or butt-mount (which would take longer to get to). It takes about 2 seconds to deploy the unit for full use.
Anyway, here are some videos. They are short and of very poor quality, but they will give you the idea. I hope you like them.
Aloha,
Charlie