If you are going to go backpacking, get a system your back is going to be happy with. $200 is not unreasonable. I mention several options below, but these are mostly canoe trip use -- portaging is a relatively small part of the trip, and enduring an uncomfortable pack for a 30 minute walk may distract you from the mosquitoes. Backpacking on the other hand, you are wearing the beast for hours at a time.
Start with a search for DIY backpacks.
I have built food boxes from scratch for canoe trips. Mostly 1/4" plywood, with internal corner braces. They were waterproof except for the lid, but weighed over 20 pounds empty, and 140 pounds full.
There are problems with repurposed items:
Many containers work well when supported from below, but are not designed to be supported from the side. They gradually deform under load. The Rubber-Maid style totes made of flexible plastic fall into this category.
A weak container that's fine on the floor may not be strong enough to hold straps. Give some thought to how straps attach to the container. Anything you can do to spread this load will help.
Other containers are strong enough, but don't fit your back well. This can be compensated by building up parts of the container with layers of closed cell foam (sleeping pad)
A 5 gallon pail can work. It's a bit short for most people's backs, You may want to cut the bottom off another pail and use it to extend the bottom. You can replace the lid with a threaded lid. Available from Lee Valley Tools, and various survival stores.
Olives are shipped in barrels of various sizes. Generally they have some taper at both ends, and have a screw on lid. The 10 gallon size is about right. Check with places that use lots of olives -- subway, pizza places, greek restaurants.
These are commonly used by canoeists to keep gear dry. While the strapping systems work, they aren't comfortable to carry long distances. But on most trips under 10% of the canoe trip is spent portaging.
Another option: You can sometimes find old external frame backpacks at used outdoor equipment places. I've seen people lash 2 5 gallon pails with screwtop lids to a frame. This is far more comfortable, but still compare unfavorably to modern backpacks.
