Those are the two pieces I have used also. You don't need the pole in-between that ads height (not shown, optional). The base with sleeve can be used upside down to add 3 inches if it is to short. Check before buying it that it's not the screw in threaded one or it can't be used up side down. I really like that for removal you simply lift the seat up, but the seat will not come out even at 30 mph. I have to ad a squirt of white lithium grease every 6 months to keep them quiet.
Backer wood is wood behind the aluminum for the screws to bite into. The aluminum is thin and would be easy for the screws to pull out of. I am not sure how to get inside the seat to hold the backer wood in place while screwing into it. Use Stainless steal screws to prevent future rust and corrosion.