1/4" T6061 aluminum usually does a good job. We were using 5/16" for the front skid plate replacement but that turned out to be a waste of metal. If the un-reinforced square area isn't more than say about 2'x2' 1/4" works fine even for heavy bashing. If you're not planning on slamming the weight of the vehicle on it it will certainly work. You can go to 5/16" or 3/8" for larger square areas but at that point you really need to consider supports and mounting more than the metal thickness.

For what you sound like your doing I'd do 1/4" 6061. It'll save weight. Just my 2 cents.

Edward