Wow. Before my time.
Ask them. Asking is pretty much free…and not asking them can cost you much more than you can afford to pay.
Meanwhile, small cake holder makers have already been busy.
Square cakes, rectangular cakes…they’ve got you covered there too.
As to upsetting Pepperidge Farm execs, it’s doubtful they’d be peeved unless they have a cake holder sideline, or would resent the idea that their packaging leads to icing scrape-off.
I’ve searched many times. Most square cake holders are 12” x 12” which is too large. Your list had one at 11” x 11”, we’re getting closer. I have done plenty of plastic fabrication and need to make my own.