I'm not sure you original question was answered. My answer is for an early +2 but likely the same for the later S models.
1) I don't believe there was a gasket between the lamp housing plinth and the bumper. I think the unit earths through the bumper. The mounting screws might achieve this to some degree. I think later advice is to run a separate earth (to reduce corrosion) from the lamp holder. I have done this and run the wire through the black cover/seal and into the boot.
2) There is a pretty thick gasket under the glass/chrome cover. This should be easy to make from a sheet of neoprene as described by others.
3) The unit isn't quite the same as an MGB.. at least the MG chaps kept telling me this when I was hunting for a replacement
plinth. My original (RHS in picture) has an extra "collar" that helps retain the thick gasket. I picked the other (which could really do with a re-chrome) up at an auto-jumble after a lot of looking. They have LUCAS and L534 stamped into them. Reproduction items are very easy to find, & will no doubt work well, but they do not look as the original items.
I'm just waiting for a couple of screws and some LEDS to complete this job....