I've been sealing them off like that for many years. It is cheap simple and very effective. Make a small hole with a punch, soldering iron or Exacto. Keep it is small as possible too for a tight fit.
It is one thing that I noticed very quickly at Danville. If the field was dry and dusty.... the grit got every where in the tank.

One of the biggest holes in the hull was the out-put shaft at the drive sprocket. The grit didn't effect the electronics and such, but I knew it was a death sentence for the TUs. The guys at NEAD and Danville that did lube well, often had grease thrown about in the tank. Just look at the undersides of the hull on your HLs & Taigens tanks. And that is just grease flung off, not oil.


BRW, I have always liked your clear covers. That way you can assess the lube condition very easily. I always try to incorporate a clear plastic section into my TU covers.