My circular old work electrical box is too small for the light fixture mounting bracket

I purchased some flush mount light fixtures to install in each of the bedrooms in my house. Originally I picked up some circular 4" old work electrical boxes (the ones with the "flags") but discovered that those are not mean to bear weight (the weight of each fixture is about 5lbs). I found the "right" circular old work electrical box which had a metal support and said it supported up to 10lbs, so I bought them and installed them. I did notice that they were 3 1/2" but I didn't think anything of it.

Now I'm trying to install the mounting bracket for the light fixture and it is too wide for these boxes… The outer screws which must go in the outer holes are touching the plastic of the box itself. I feel like I'm screwed now. If they make bigger electrical boxes, I could buy them, but I already have three 3 1/2" holes in my ceiling so removing them and expanding the holes will be a nightmare. These fixtures are meant to match others that I was able to install in original electrical boxes without a problem, so I really don't want to change them.

This is the light fixture:

This is the electrical box:

And here's the problem:

Any help is appreciated.

Edit: I just tried a 4" round old work box (with the "flags") and it has the exact same problem… Is there a different type of box that I need to be using for this? How would you make this work in an "old work" style situation (E.g. I have a 3.5" hole in the center of my ceiling)?