Different units have different roof construction so a "fits all" answer doesn't apply.
I'm 250 # and walk on my Flagstaff with no problem except where the A/C ducts are. You can feel the roof give slightly when you get over one of the ducts that are channeled into the roof.
On a SOB unit I did like Mr. Havercamp and used 2" foam blocks about a foot square.
I made four of them, put under my hands and knees and crawled around.
You could always buy a pair of these...