Do breaker panel fillers need to be CSA approved or simply 'adequate'? 2-202 (2) is the only rule I could find.

I would contact the electrical inspector if you have doubts.

Anecdotal story somewhat related to your question...

I had to quickly come up with a method of isolation before attaching a terminal block to the inside of a transformer housing. I used a thick piece of Bakelite out of an old electrical panel that was used for isolation, cut it to size, asked the technician who was installing the equipment that the transformer was powering if it was a satisfactory install (he said it was), fastened it to the metal housing, and attached the terminal block. Since it was already material used for a similar purpose I didn't worry if was an "approved" application.

BTW, 2 comments; (1) I would never use Bakelite again in this fashion since, after doing some research, there is the possibility that it contains asbestos which would have been released into the air when I cut it using a radial arm saw, (2) it really stinks when you cut through it and everybody in the shop let me know it.