I'll second the advice above about using Lexan. It stands up to impact well.

I wanted an enclosure for the mill but it was quite expensive to get one from the manufacturer. You can design your own and have it cut and bent at a local sheet metal shop. I riveted on U shaped channel on to flat panels that hook on to the lower splash pan. I never put a top on it and need to install some doors also on the back side of the mill. The price was not too bad I'm also in Canada. Mine is made out of galvanized steel and will last as long as the mill. The back and front are bolted to each other. Find a local sheet metal shop. For them it will be an easy job.

The back part in the middle will eventually get sliding doors. For now I am using plastic sheets. Held in place with magnets. The stuff they use for vapour barriers. It works fine. If you need a top and want to save money you can use a sheet of plastic.