I think the field mod of the box cover is OK, I do it from time to time. I also have a shop make custom boxes and covers from time to time. They have all the punches for all kinds of devices too. Usually for controls and such. Only a few types of boxes say listing is required, like floor, heavy fixtures or fan boxes. But does say the box or cover needs to be listed otherwise, and the construction spec's are in 314.40. 314.41 covers... covers, and take note that it says "OR", not "and".
Metal covers shall be of the same material as the box or conduit body with which they are used, or they shall be lined with firmly attached insulating material that is not less than 0.79 mm (1/32 in.) thick, or they shall be listed for the purpose. Metal covers shall be the same thickness as the boxes or conduit body for which they are used, or they shall be listed for the purpose. Covers of porcelain or other approved insulating materials shall be permitted if of such form and thickness as to afford the required protection and strength.
Anyway, it does take a lot of Ca Ho Nay's to show your own work, knowing it will be criticized. Kudo's to you!