I used to adjust the diff a lot on my racing Imp, which was easy. Like every time I broke something, which was often. But it only had to last until I broke it again, so it was very agricultural
The engagement is adjusted both by shimming the pinion in and out, and by moving the crown wheel side to side. Every time you adjust one, you have to check/adjust the other. There's about six pages in the workshop manual on diff adjustment, and it doesn't look like a job I'd want to get involved in with the Commer, even if I had the special tools.
A useful guide is to use a bit of engineers blue on the pinion teeth, and see how it prints onto the crown wheel, or even just look at the wear pattern. There's a chart in the manual. If that's near enough OK, and there's just a few thou of backlash, I would vote to leave well alone. Another good guide to the state of things is to see if there's any side to side play at all in the pinion shaft with the oil seal out and the nut tight. Any play at all and it needs doing. You're supposed to be able to measure the torque required to turn the pinion, but as long as there is just no play, that's OK by me.
Here's a photo of the teeth on my diff. Although it's a crap photo, it does show a half decent engagement pattern (the shiny bits on the gear teeth). The pitting is due to the pinion nut having been loose when I bought it, and the pinion was being drawn too far in on the overrun.
[edit] Here's the same photo with the engagement pattern highlighted.
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Oh, and in answer to your original question, I just made up a paper gasket and used a bit of RTV silicone as well. It's been fine.
