I use an Army surplus medium size ALICE pack. They are readily available at Army surplus stores and Flea Markets. Prices vary considerably so talk it down for a good deal.

I have to agree with Gary, an A.L.I.C.E. pack is a good investment, very rugged and hold a lot.
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I have had mine about 15+ years and the only problem I have noticed is the snaps on the pockets can come apart in sub freezing weather. I was deer hunting in 10 to 20 degree weather when two snaps popped off (actually fell apart) but it was not a problem since the strap is long enough to tie down. Now I unsnap all snaps and tie the straps if it is below 32 degrees. So unless you are in North Korea in January doing something you shouldn't then you probably will not have a problem. The ALICE pack is a good value and the frame can handle about anything you can attach to it.

If I am carrying more than a hydration bladder and a detector, a med ALICE pack is my go to choice. And I see it is the favorite of others as well. More than enough room for everything I use or need in the field short of bringing in bigger equipment. Pans, classifiers, cracking tools, sweeps and just assorted stuff along with clothes, food, water and emergency supplies all fit nicely with a bit of room to spare.

If you want a good quality backpack go to a mountain outfitter store and spend a few dollars. If they don't custom fit it to your body walk out the door. I like Dana Designs backpacks and I use an internal frame one. I can carry tons of gear including shovels and have zero issues with my back or shoulders because the pack was fit to me properly. I highly recommend this if you are serious about getting a good backpack. If you skimp you will feel it as your hiking along half a day and wish you didn't buy a off the shelf model from Cabellas or something.

Tim, I am looking for something like what you are talking about, so far I have come up with a "A.L.I.C.E. BackPack Frame" from Amazon which is just a frame and the shoulder straps etc., from there I will make a folding shelf with some plywood and hinges made to fit what I am carrying. My shelf will have sides that come up about 4-6" to help secure my load.

I hope this helps. Don't forget bungy cords and mini ratchet straps.

You guys are gluttons for punishment with those packs hehehe.. but if it works for you that's all that matters.   Dana is out of business shame shame best expedition stuff made... Anyhow there is a reincarnation of sorts if anyone is interested in quality gear that is usually a one time purchase. 

 

 https://gearjunkie.com/mystery-ranch-backpacks

 http://www.mysteryranch.com/

I have an Alice pack, I use it for sampling trips where I am just panning, I have a hunting pack frame with a shelf that I got for hauling my river sluice, buckets, pans, waders, metal detector, pick, shovel... Everything. It's heavy but it beats the hell out of carrying it in my arms.