I used a hook tool from NAPA rather than a cotter pin puller tool. The cotter pin tool I have has a sharp point, and you don't want to puncture the rubber. I used the hook tool to work the window into the rubber weatherstrip, after installing the weatherstrip into the wall opening, but this was on a Uhaul
camper. We used to install truck windshields by putting the rubber on the glass, then rope around the outside of the rubber, set the windshield in place, and by pulling the rope on the inside of the cab it would set the rubber into the opening. I did not use this method on my Uhaul
window, but did use dish soap as a lubricant to install the lock strip. I'm not sure exactly what rubber trim you are using so what I'm describing may not be applicable to your situation, but if it has a separate lock strip the tool and soap work good.