Scamp sells the seal for the non opening plexglass windows. Since your trailer is about 6 YO and you are missing a few caps you should replace them all at once. Scamp sells cap kits for specific size trailers to replace all the caps. Or you can go on line and order directly from the vendor of you know what size you need. (Pro Dec).  With a older trailer and dealing with Scamp, Inspect the trailer for any needed parts and order them all at once from Scamp. It's best to call them and have the parts shipped together to save on shipping. Scamp also sell specific rivets along with a variety length pack of rivets. 5/8"x 3/16" rivets and shorter usually can be found locally. With rat fur and longer rivets Scamp is a good source unless you want to buy in bulk..

Eddie