Portable Garages - Poor choice!
I wouldn't give the portable or canvas garage a second look. They WILL NOT stand up to any wind. Even the lightest thunderstorm will usually take them out even if you tie them down securly. And if your project is in the structure at the time...it will be damaged. I'm speaking from experience of my own and a buddy of mine who lost his "garage" and lost a really good novice paint job when the thing crashed, AND IT WILL!
Now if you just use it on nice days....well maybe it will be OK..but usually that's not the case. I built a structure out of PVC and corrugated fiberglass that I'm still using (as a greenhouse now) to this day. It has survived for about 15 years. And it was easy to build. Build it like a Quonset hut, the small diameter PVC (I used 3/4") will bend easily and the corrugated fiberglass attaches with self taping screws quite easily. The whole sturcture cost me less than $200 and took less than a weekend to construct.
If you need more info...let me know!