You guys seen this, think its good enough?

http://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=331345960144&category=53298&pm=1&ds=0&t=1462398059727

http://motoform.blogspot.se

If you are prepared to lose the transmitter... 

what?
explain please.

http://motoform.blogspot.se

Those devices use a transmitter that emits a certain frequency, so you put it somewhere inside the cavity and listen from the outside for where the frequency leaks out. They are made for finding leaks in places like garages or your car, so you're talking big leaks that don't make any noise of their own. 

I built a very effective leak detector. (Don't laugh it works well)
Take one of the cardboard center rolls your cloth or plastic came on.The 2 inch stuff will do.
Cut one piece 15 inches long another 5 inches.
Now cut a 45 angle on one end of each piece.
Join them together with duct tape.
Hold the small end up to your ear and go around the bag looking for leaks.
It took me like 5 minutes to make while my bag was leaking so I made it in a hurry.
You will be amazed how loud the leak sounds with this detector.
I also have a doctors stethoscope which does not work as good as the card board detector!
Patent Pending
Fred

That will work for holes in the bag, which are quite loud leaks. For tiny leaks through the bagging tape you won't have a hope. But honestly, even big expensive leak detectors don't work for that most of the time. 

I've seen some university students using ultrasonic leak detectors during vacuum bagging a racing car floor and rear diffuser.. although the leak detector did not find any leaks (the workshop was noisy) they still lost vacuum. But it was useful in finding a few leaks though. I guess it would work in 60-70% scenarios. 

Negative, I just used a 40khz transducer. You want the receiver not the transmitter, although I believe a lot of them are now combined units.