Yup, It's a little like jacking up a car so you can crawl under and work on it. You need to be sure of what you are doing, but it is done millions of times a day around the world, successfully. That being said, no the seal between the can and the doesn't have to be so tight, just close enough so the jar won't fall off.
Here's a little more info. The one on the right is where this project started. It’s really good for camping, hiking, backpacking. I was using my experience making tlud camp stoves. I just made it with my jack knife. I was amazed at how well it worked, though later ones work even better. The one on the left is just a longer burning version (~36 hrs) of the first one I showed you. Lots of different jars will work, as long you can find a can that fits closely.