Well, what is this for? XJ, YJ, TJ, etc? I'd just copy an existing mount. No need to reinvent the wheel.
To keep it from walking off, you could do a few things. Spot weld all the bolts and nuts in place or you could weld U channels over them. Some people take 1" lengths of tube but you can still use a socket to get around those. Another idea is to use non-standard fasteners. Large torx or allen heads, etc...