Several things I want to note:
One is that the unit will have a rating plate. The link you provided was for the series, which can be any of several units from 3/4HP to 4HP.It will state the rating of that particular unit. That is the max HP you can run on it. I tend to stay a bit below that to avoid overloading.
Putting a controller in a sealed box is a bad idea unless there is sufficient space for the unit to get rid of heat. Generally, the bigger the box the better.
The vent holes in the controller case are there for air flow to remove heat. If the heat is removed, but contained in a box, you basically have the controller in an oven. It won't last well that way. Either have a very large air space, a fan moving the air around inside the space, a box that is conductive for heat ( metal box), a filtered air flow through the box, a heat sink, or a combination of the above.
A box like you show in the link with ventilation holes, a small computer fan, and HEPA filter elements over the vent holes works very well. An alternate method to assure a long life is to mount the controller away from the metal dust creating area and have remote Start/Stop and Speed controls at the grinder. These are easily added and are standard options covered in the operations manual.
Another option is to mount the controller in a metal box and put the unit against the back of the box. Put a heat sink on the outside of the box back.
Whatever box type you used, cutting out the front panel and putting on a Lexan/Plexiglas cover makes viewing the controller easier. Use silicone adhesive to attach it.
One of the simplest boxes to use is a clear plastic storage box with a snap on lid. I like the ones that are the size of a shoe box. Mount the lid on the wall, mount the controller on the lid, and snap the clear box over it. To adjust things, just snap off the box, make the changes, and snap it back in place. A small fan can also fit in the box easily.
