Can you help identify these clips (or a replacement)?
Make a new piece out of epoxy putty
You can get some of the two part epoxy putty and sculpt a new piece that matches the old one.  Cut it with a razor and put the screw holes in when it is soft.  Epoxy putty is really hard when it sets.  It should be able to do the job that your old piece did.
There are lots of things on the web about making things from epoxy putty - flying ladies, chess pieces etc.  Your needs are a lot more straightforward than that.
Maybe make a spare while you have the putty in anticipation of the other plastic piece breaking someday.