They do make some that are sealed. The average joe's though are not sealed perfectly. The sealed ones I have seen are white or cream colored.
I just got done finishing the rest of my way covers. Here are a couple pics.
I just used some .063" thick roofing rubber. Over the X motor, I just bent a little polycarbonate. Bent up a little aluminum for the Y. As long as you leave a drip edge on the sides so the coolant will drip off, you are probably okay. Both of these are open on the bottom. I am running 203V's though and these motors hardly even get warm. They don't get hot.
