Are you sure the leak is coming from the gasket, and not say the operating shaft? Sometimes it can be really tough to see oil running down a vertical surface, and much easier to see it dripping off a low point.
When you get the cover off, check that the O-ring is in place. It's easy to overlook. Also examine the gasket carefully (since you used Hylomar, it should still be intact).
Remove the studs and check that both mating surfaces are perfectly flat. I like to lightly rub a flat, fine tooth file (aka machinist's file) against the surface just enough to polish it up. Any low spots won't get polished, any high spots will show up early. There's almost always a high spot or two around the stud holes. (If you are ambitious, it's actually good practice to put a small countersink in each hole to prevent getting a high spot there.) I think I see some small damage to the edge of the cover in your photo, a tiny groove near the front edge. There might be a high spot at the end of that as well.
Reinstall the studs, and spin a new nut down each stud in turn. It should turn easily all the way down the threads. Double check that the space between the nut & OD body is smaller than the thickness of the cover. Do the same checks with the two bolts. Don't forget new lock washers when reassembling, nor the flat washers on the two bolts.