You know those plastic sleeves for trading cards? A GB game fits rather well inside, and you can tape the back to keep it sealed. For GBA games, you can fit two in one sleeve.
Systems: SNES, PS1, GBA SP, Gamecube, DS Lite, Wii, PS3 (Fully Backwards Compatible), Dreamcast, Nintendo 64, Sega Genesis, a Sega CD, and a Sega Saturn.