[elhigh]

Run a few tanks with the holes - being as consistent as possible, don't do any testing or jackrabbit starting, just drive, if possible, like a robot every day - and then drive a few more tanks with the holes closed up. Tape them from the inside, of course - it looks to me like your finish on the back is pretty good, and taping it over from the outside will just look awful. Plus taping from the inside, pressure will tend to force the tape on, instead of off.

Then fab up a quick-and-dirty pan to cover the back end and smooth things out approaching the rear bumper. Get a few tanks in that way.

I'm curious to see how the different approaches shake out against each other. I don't think it's the best approach but I'll give you this: it looks well done.