Hi Terry,
I made a mylar one for my Mig 15 using CA as I described above. It had a 1/2" overlap seam which I roughed up with fine sandpaper and used the CA sparingly. It held up fine for about 100 flights, until the Mig met its demise. The problem I found with packaging tape is that the exhaust residue tends to migrate under the tape and the adhesive gets soft.

Donnie,
It doesn't have to be any thicker than a sheet of paper. The stuff I used was only about .007" thick and worked well. I was able to find mylar in various sized sheets and thicknesses at a place that sold plastic materials and supplies for industrial purposes. Check the business Yellow Pages. When I told the guy I only wanted two sheets and what I was using it for he just gave it to me. Probably wasn't worth the trouble of writing up a $3.00 invoice. If you can't find it locally try searching online.

The original Byron thrust tubes were made out of lexan. They got brittle with age and didn't last very long. Mylar is a lot stronger.
Joe