That’s a sweet looking bench. Did you MIG or TIG weld it? As for optional top material you might look at Cedar boards. They are likely the lightest weight, easily located at a big box store, wood material and weather resistant boards. They could be tongue and grooved, biscuit joined, dowell Ed or edge butted to make a beautiful looking and stable top. Only down side is the wood is pretty soft and could get dented easily if that was a concern?
You might also,consider a drummer’s throne seat. It’s very comfortable and the threaded post is easily adjustable for different heights if you ever change your rifle rest set up height the seat is easily screwed to the right height very quickly.