I, likewise order an new Astrodome cover last year from Airstreamdream for my 64GT..
The old saying of measuring twice before cutting is so very true...Except, in my case, I must have measured 4 or 5 times, as I was wanted it to be "perfect".
The real challenge was the replacement screen..I had nothing to judge by except for the old screen over the rear exhaust vent..I had special ordered one from a local hardware store.~ Cost was way below what I had figured..(At least I don't think I could've ordered it any cheaper and have coming out as nice as it was).
Here's some pictures of our efforts..
(Starts at entries 111)
Good luck with yours~!!