I was going to stack 5 pieces of 1×8, glue together with construction adhesive, and paint. I'd then be using longer bolts to install it.  Am I missing an easier solution? Im basically building a 12" by 7" by 3.75" tall rectangle to mount it.

Why?

You can buy a piece of plastic sheeting the exact measurements. Probably cost more than making it yourself but will never rot.

@slonezp Thanks for the reply - where would i get the plastic sheeting? Believe you me, Im looking for the easiest way to do this.

You should be able to find something if you Google it. Plastic sheet or plastic block. 

Looks like I'll be in for about $120 plus whatever the longer bolts cost me. Boat Outfitters has it but the thickest I can get is 1".  I'll glue four together and countersink a few screws up the middle.

A cheaper option might be plastic cutting boards