While his picture shows where the retainers should be placed, you don't necessarily need to screw them in.
On my cab, the top panel is about a half an inch lower than the top of my sides. My top marquee retainer fit very snuggly in that area, so I did not fasten it at all.
I used some fast setting epoxy to fasten the bottom retainer. Although I just happened to have this laying around, you can probably find a small tube of this stuff at a hardware store for a $1.50 in the glue section. I had to hold the retainer in place for about 5 minutes while the glue set.
This is a little off topic, but someone out there may be wondering how to cut the retainer to size.
I used a circular saw. I put the retainer on the edge of one peice of mdf, with the curved part hanging off. Then I put another sheet of mdf on top of that one, covering the entire retainer, forming a wood and plastic sandwhich. Then I cut and I think it worked really well, giving me nice clean cuts.
Good Luck!