Simplex 4098-9788 Base Mounting

So I recently purchased a 4098-9788 2-wire conventional base for my new (to me at least) Simplex 4098-9601 TrueAlarm detector. Unfortunately upon trying to mount the thing, I noticed that the base mounted crooked instead of being completely flat on the box (a standard metal octagonal box that’s surface mounted on my demo board) and while it was fine when I had the head on it it would move when I tried to move it because the entire base is not on a flat surface.

Unfortunately I didn’t know this when I bought the base but for surface mounting all detectors require a 4098-9832 trim plate to mount correctly.

However, I was just thinking of going to Lowe’s or something and seeing if they have a metal octagonal box cover plate with a middle knockout in the center that I could just attach to the box using that, and knock out the middle knockout and then mount the base on top of it so that the plate acts like a trim plate of sorts so that I don’t have to buy one off eBay and wait a week until it actually gets here. I mean I could just mount it the way it is and have it be crooked (after all it’s just on my demo system) but I don’t feel like putting stress on the plastic base that the detector uses anyways (it’ll mount to the box fine and stay up there but it will be crooked a bit).

I’m assuming this would work the same way as the trim plate would because all it would do is just provide a flat surface for the base to sit flush on so it doesn’t morph the plastic, but I’d thought I’d just post it here anyways to see what you all might recommend.