I use tupperware all the time.. the thing with metal is that it's hard to get the soap out at the end, your moulds do need to have a bit of flex and, unless your Superman, metal just isn't going to do it for you.
I also use food product containers, yogurt pots for instance... if they can go through your dishwasher, you'll be able to put M&P into them. Think of plastic bottles for your lotions etc... cut the top and bottom off them and you've got a perfect tube mould (secure 6 layers of cling film to the bottom of one end so that the soap doesn't pour through.
The silicone mould and slab moulds aren't hugely expensive and your local food store will probably carry them... experiment, experiment, experiment is the key!!
And then show us pics!!!!