I just used a Pelican 1500 for Pine Hill Cave in southern ky (lots of crawling and tight places) ... I drilled into it, though, and mounted common drawer handles to the sides to use 2" webbing straps so I could "wear" it like a backpack (I used silicone sealing adhesive and then treated the inside with epoxy resin). I was able to turn it around and wear it on my chest when crawling, and I just drug it through places I couldn't wear it through. It really wasn't too bad, and I carried my 5D2 with a Sigma 12-24mm lens, attached, a 580EXII, two 160-LED light panels, a cheap 48" tripod, and a ton of batteries.
Packed:
Detail of the handles I installed (secured with #10X3/4"32 machine screws and nylon threaded lock nuts - tapped #8 holes into the case so the screws "threaded" themselves into the case material, sealed with marine silicone adhesive, then epoxy resin over the inside):
With the contraption on (no jokes about the model, haha):
And here's what it looked like when we got back:
I really love Pelican cases after this trip. That thing was half-dunked a couple of things, drug through water and over rocks and mud, it fell once, about 4-5 feet to the rocks below me when I tried to pass it off to someone, and by the end of our trip, I was literally slinging it in front of me across the rocky ground through a belly-crawl. It really took a beating, and everything inside came out perfect! It's a bit heavy, but it was worth it. It did great and I was happiest to have installed the straps during any of the crawls - there's a particular one that is about 100+ yards of just hands and knees on not-so-smooth rocks and 4 inches of flowing water - had it not have been for the straps, I'd have left it behind.