Yeah i got some black u-channel (2mm) from a rubber store. The store primarily sells Rubber/Plastic pool equipment and rubber flooring, I'm sure you probably have something nearby that's similar. It's really really cheap when you buy it from these kinds of stores. Rather than relying on PC modding sites that may take a while to ship + added cost etc.
Edit: These kinds of shops also sell grommets, O-rings, Big chunky black rubber chair feet, walking cane feet, non-slip perforated matting. etc. etc. etc. Amazing what you can find once you start looking away from typical PC-mod shops and start wondering where these things came from originally.
You can get a pretty clean cut with flat files, but it still needs some paint on it to look clean. As you can see from this pic, while it's somewhat straight it's still missing something. Also use a precision file to clean it all up and make it smooth as a babies bum. Also you may want to consider rounding the corners unlike the below pic, if using U-Channel. It's hard to get U-Channel to stick when you have 90 degree corners like below. (unless you cut it into 4 pieces)
Precision files.