Strong, waterproof bivvy bags

Hi, I don't usually post here as I'm not really into camping per se, but I do volunteer as a dogsbody for SARDA (Search and Rescue Dogs Association) which means I spend hours on end on cold, wet mountain sides waiting for trainee dogs to (hopefully!) find me.

We have Buffalo bags for warmth if needed and some lightweight bivvy bags that provide some degree of waterproofness, but they aren't great at that, plus they're rather flimsy and over time develop holes in them as we're often lying on ground with heather, gorse, rocks etc., but also from boots when we're using just the bivvy and not the Buffalo bags.

So, can anyone recommend a good heavy duty (weight not really important as we're not carrying them any distance) waterproof bivvy bag for our purposes?