I've got access to some very large used catering size food oil cans. They are quite colourful and thought it would be fun to grow some veg in them. i've seen this done in South america. But I'm at a loss as how to remove the lids, as at present all they have are holes punched in the top to get out the oil. I'm sure my normal kitchen can opener would not cope with the thickness of the metal. Any suggestions?