PeterH:
I wasn't thinking of a weighing solution - just a preloaded mechanism that would yield slightly when the target weight was reached - you could think of it as linear force version of a torque wrench. It would just need a spring with a stud through it to give it a preload. It's only carrying half a ton or so - it's not going to need anything heavy.
i see what you are saying, the only issues is moving and bouncing around i think could cause it to trigger to early, and it would have to be under the weight.
GoForSmoke:
Suppose you filled your bags from a tank/barrel with known volume?
i will be pulling from the lake, its actually for a boat so i don't think that would work.