This little mod is great for cans you'll use up in like a week.
Btw, tin poisoning only really occurs when you leave an open tin in the fridge for a few weeks with something acidic like pineapple. Please don't school me on open cans in the fridge, I just don't care.
You will need:
a can opener that doesn't leave the sharp edges
Pic 1: Open tin
Pic 2: Wash lid
Pic 3: Make 4 10mm cuts into the top of the tin.
Pic 4: Turning the cleaned lid upside-down and starting at one of the cuts, work the lid over the tin.
At the finishing point you should be left with one cut. With your finger, push the tin so that one side of the cut crosses over to enable the lid to have a nice tight fit.
If you're not sure the lid is on properly, place your palm on the lid and rotate, you should be ale to feel if it's in place.(this will also help the other cuts to overlap each-other making it easy for next time.)
Pic 5: BOOM.