Here's my budget Roll off Roof observatory. Nothing flash and I'm certainly no builder but it does the job..
I used this shed from
Vidaxl which was $356 delivered. I followed the instructions to get the main structure together, walls, doors, then just made it up as I went with a few sketches of what I had in mind for the roof.
I was inspired by Peter's (pmrid) shed in this
thread.
Hope I haven't broken some forum rule by putting those links in.
cheers
Mark