I lined the inside with 3/4" plywood to add rigidity to the storage box. It's screwed from the bottom-up. It's also more convenient for tying things down such as the battery box.
I needed to raise the storage box about 1.5" to clear the existing frame angle iron which is used for securing the battery. So I used 2 oak rails (painted with Tremclad) and bolted them to the A-frame. The box is then screwed into those.