I apologize if I don't use the correct electical terms in the post.

I recently purchased a Heath/Zenith motion detector (model SL-5511-BZ-B
if that helps)light. The base of the light fixture that connects the
the wall is circular and is about 4 inches in diameter.

My junction box in the wall I guess is 4" in diameter as well. It has 2
tabs on it bent inward to connect, I guess, a backplate to attach the
fixture and ground wire.

The light fixture's base has 4 indentations of where a hole would most
likely go, top left/right bottom left/right, but there are no holes.

No backplate was supplied with the fixture. So I am using the existing
backplate. And it's not the right one. The tabs of the junction box
are at the top left/bottom right of the box, like the \ in an X. The
backplate is attached to those tabs. There's another bar, about 1/2" in
diameter that can swing around.

My problem is that I drilled out two of those holes to match the \
position on the tabs. If I connect the top left hole to the tab, I am
way out of position for screwing in the 2nd screw. Can't be done.

What do I need to do to attach the lamp fixture to the junction
box/wall? Do I need a special backplate? What do I do about the
fixture's base that has no holes (but does now because I drilled them
out).