WAY easier to pin it while it is open.
Align the holes in the non-motor side cap with the hole in the axle and slide a pin or long nail into the axle securing the non-motor (spring) side.
Then simply remove drill out the pop rivets holding the motor on; unbolt the arm top; unplug the motor and slide it out of the tube.
MARK THE LOCATION OF THE MATERIAL ON THE RAIL AND TUBE WITH A FELT TIP MARKER.
Remove the two screws securing the material to the top rail and then you can slide the old material out.
Slide the new material in (watch out for sharp edges where the screws penetrated the rail!) stopping at the old locations; replace the screws.
Reinstall the motor; remove the cotter pin and you are done.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW