Fastening to an old-work box?

Tom738

Newb question here.

I picked up a plastic old work box to install a new wall light on existing sheetrock. When I went to install the light--which you do by screwing a round metal mounting plate into the box and then screwing the light into the mounting plate--I found that the old work box doesn't have threaded screw holes, but has smooth holes ("splined,") as some web sites seem to call them.

So how do I connect a light to the old work box?

joecaption

Hope this is a really light fixture.
The screws will still go right in and make there own threads.