Sorry I didn't respond sooner.
Chances are you already have the answers you were looking for from me. The trash cans that several people have used here are ones you can get at Wal-mart. I think I got one or two for $12, maybe 15. I don't know how long you'd have to heat it to cut with scissors--I think you'd probably kill your scissors trying. You listed 30 secs above, so maybe someone answered that for you. I used both a craft knife (takes a lot of patience and care not to snap a piece of plastic and not cut off your finger), and tin snips (which might be an item you'd have to buy, or one might be hiding in the garage--your folks might already have one).
A few Mandos here have used wraps as part of their outfits--they lend a certain look to your gear that you should consider as a whole. As I said to someone else recently: remember that Mandalorians are soldiers. What is the purpose of the wraps? Soldiers in the past have used the equivalent of wraps on their lower legs--they're called gaiters and are used to close up the bottoms of the pant legs over the tops of boots, covering from ankle to mid shin or below the knee. I found two different versions on military surplus sites.
As you've probably found, your biggest investment will probably be your helmet. I have two Rubies Jango helmets. I'm modifying one now because it's too small for my melon--it might fit you well. But they cost $60ish. Watch those ones on ebay. A few others have made helmets out of all sorts of stuff, including cardboard. It can be done. For the helmet, though, I would consider saving up or asking for the money or a helmet you like for a gift. You can certainly make your own--we call them scratch-built here--but they take some work.