Way back in the 80s, I did some tinsmithing and made stuff out of tin cans, copper, and aluminum flashing. You need to leave extra space for seams, because you double fold them, then hammer them down. you can also punch designs in the metal for candleholders and such. It's fun, but lots of sharp edges to be aware of.
I think the only problems you may run into with outside use will come from oxidation - rusty stains will be hard to paint over in the future. Deoending on where you live, you may want to protect your quilt block with a polyurethene or spar varnish. Sounds like a great project - we'll be looking for pictures!
Another thought on weatherfastness - Home Depot carries fascia in white and dark brown around here, and if you have a tin roof supplier, there are beautiful colors available. I think I've seen some other likely ingredients at Tractor Supply.