I realise this was posted almost a year ago, but thought i'd comment anyway for the help of others. To stretch the canvas without corner wedges, first wet the back of the canvas with a damp cloth. And then dry it with a hair dryer and repeat the process. It works on the same principal as when you wash cheap jeans they get tighter. Same idea. Hope this helps.

better than a minotaur saw!

if you need a stretcher to carry someone in an emergency, you can always use a ladder. If you haven't got a ladder, then you'd have to make one! I wouldn't know where to start! Two large coats with poles through the sleeves make an emergency stretcher too