I myself have the same question as just purchased a 2nd hand safir cp9 and new frontier stove + 3 flue extensions. I have seen some that have crafted it from perforated metal and then bolted the long edge together and folded the ends down to the size of the original flue to make a partially closed of cylinder (if that make sense). I have taken a look at images of the tentipi stove they make and they hang their shield from the top of the flue on chain rather than bolt it in place.
However my (crazy) thought is can a welders blanket be wrapped around in the correct place as all you are trying to do is stop it burning the tent. Found places around ebay etc that are cheap so could always be a possiblity.
Maybe someone will post a tell me this is stupid idea, if they do I will be making mine this weekend out of perforated sheet so will keep you updated.
PS Have you thought about a spark arrestor for the top, wire bird feeder apparently works well.