I designed [this simple NES cart cover][link] to use as little material as possible, but still be robust enough to print even on janky printers. Hopefully this will help keep your NES carts safe and sound for future generations. It’s available [on thingiverse][thing] for the low price of FREE.
[link]: http://thisoldconsole.com/post/146951201585/original-nintendo-entertainment-system-cart-cover “NES Cart Cover”
[thing]: http://www.thingiverse.com/thing:1659496 “Nintendo Entertainment System Cart Cover (Tender NES)”