All of clear gear case covers that I made, I just used a washer and nut. In the grand scheme of things, you are not really going to notice them anyways. Biggest thing at a quick glance is that the gear mesh is good.
Such a cool and unique build! I cringed when I saw the first video of the speed test. Glad it stayed on its tires! 8) I remember seeing someone from RCCaraction building nitro crawler type truck that was pretty cool as well. Hard to tell in your pictures, but are you running power to the front axle ...