I was getting frustrated with my mat constantly flopping onto the floor before/after cuts, so I made these extensions to bump out the tray just enough to support a standard 12x12 cutting mat.

You can print and many or as wide of clips as you'd prefer. I have two at 15mm wide which is more than sufficient, but I may print a 3rd for the middle. The fit around the existing tray is pretty close but there's no actual attachment mechanism so I used a tiny bit of sticky tack on the front of the existing tray/inside the extension to make sure they don't get bumped around.

Note: you cannot open the compartment in the tray with these clips on, so don't attach them permanently if you want to use said compartment.