Here is a link to a custom made vacuum valve by Miso at Vivid housing. It cost 65 Euros. I have been using it with a small aneroid barometer to monitor vacuum inside the housing. I had to shave some material from the back of the barometer, to make it fit loosely against the housing wall. I made an inexpensive pump that works very well to apply vacuum.
I have used this on system on two trips with good results. I had no problems rolling off the side of a Zodiac with my rig in hand.
Albert Kok was going to test the use of a very small electronic mini altimeter. Maybe he will let us know how that worked out.