Evaporative Emissions

I drive a 2000 4wd Dodge Ram 1500 5.2 V8 with a major evap leak according to the trouble code. I checked all the hoses and they seem to be fine. The only potential problem that I can find is that in the rear of the vehicle there are 2 canisters. The more rear canister is round and the forward canister is cubical w/rounded edges. They are attached together by a hose. Anyway, the forward canister has a lid on the bottom that was loose and with a little tug it came right off. Could this be the source of the leak? The lid won't go back on with enough authority to convince me it isn't just going to fall off if I drive. How would I go about affixing the lid and getting a good seal? Do I have to replace the entire canister?