In situations like you describe, I have found a butt pack (military style, not yuppie) works well. As opposed to a backpack, you can keep it on while working (so your gear is handy, and doesn't get stolen), and it doesn't interfere with arm movement. www.cheaperthandirt.com carries surplus ones, the web belts, and Y suspenders if you want the weight distribution. I usually strap my rain gear to the bottom as the built in straps are long enough to do so. You can add canteens and other web gear as needed.

All of these items are available new at substantially higher prices from the usual tactical stores.