I did my near side one this summer, I was ready to go down the foam core insert route but the slider had corroded.
Remove the slider, degrease, mask off the fixed window and use a new Stanley blade to cut the edge of the masking tape for a clean edge.
Re-assemble, final degrease and use Soudal Fix All high polymer adhesive or similar.
Smooth in with a wet finger and avoid overworking.
Remove masking tape asap, leave any mistakes until dry and remove with a new Stanley blade