-Cut a piece of 16 guage steel the same size as the armrest plywood- drill holes in it to attach it to the plywood. (If you're starting from scratch- use the nuts I mentioned, or if the armrest is intact, use screws)

-weld three tabs on the back end, one to fit vertically into the notch in the rear of the battery box, and two horizontally (one on each side) to prevent the cover from popping up.

-weld the forked piece at the front, to slide behind the 1/2" bolt and approximately 1" square block at the front of the box, which is then tightened to lock it all in place.

Mel came through with nice pictures. But the factor arm rests all had a piece of steel with the wood Mel has on top of that. Held together with the special nuts Lazyts was talking about. Mel also did not show the nut used to tighten the front down, not that I have ever found one that had been tight. The nut is a square piece of 1/4 inch about 1.5 x 1.5 with threads taped in the center.

With better plywood Mel's way looks real good to me. Did you put that together your self, I have been doing my own with staples or roofing nails for 40 years. But I always use a lighter color than black. I may not be the brightest, but I only sit on a black one once on a 100 degree day an we get those often in summer.

 Whats the matter Ray you don't like solar seat heaters? in the summer time you can always resort to the old standby,  A flake of hay double wrapped in a gunny sack and they are adjustable, want to sit higher grab a thicker flake and you won't sweat against the vinyl works pretty good as long as there are no goat heads in the hay