I did the same thing a couple weeks ago, I ended up with a different brand but almost identical battery switch.
I took out all 4 knockouts and then dremel'd off the remaining plastic "feet" at the corners to make it shorter to fit under the lid of my smaller battery box.
Didn't think there was any point in putting the bottom plate on the switch as there is nothing exposed that wouldn't normally be anyways.
I didn't have a hole saw the right size and ended up with a hole that was sligthly too large, so I used the extra gap to actually JB Weld the battery switch in place. The dark grey color matched fairly well.
The underside of the switch rests almost perfectly on the battery so there is no movement, and the JB Weld epoxy has made it all one nice uniform piece molded into the cover.