I recently finished an awning screen which is a bit different than a fitted sliding door screen, a bit more ambitious of a project (blog post with material sourcing here
I plan to also do a separate fitted slider screen for those times when I need the ventilation from the sliding door, but I'm not staying put long enough to bother putting up the entire awning assembly. But I haven't gotten to that project yet.
If you go the route of DIY, you might want to check out the noseeum fabric that I reference in that blog post above. It's cheap and quite good quality, and packs down amazingly small because the gauge is so fine. I would never want to stuff a conventional heavy mosquito screen into any Class B.
I also used this noseeum fabric to make a fitted rear door screen, which is held on using neodymium magnets (blog post here
). The issue I've heard with Velcro attachments is that, if they get wet, they tend to stay wet, and hold that wetness in undesirable areas - right on the steel door frame.
We have a Yeti cooler on our hitch carrier which blocks the opening of the driver's side door much of the time, so this particular rear screen was constructed to be only one door wide: