Here are a few photos of factory hex bolt and bolt installed top of my bolt had 8.8 on head if any help. Grandson used a 1/2" wrench to tighten the bolt
In first photo you can see the front factory hex head bolt and the bolt I installed on back of seat with the 3 washers between seat bottom and seat frame. Depending you could use 4,5,2,1 or what ever amount you find comfortable. I may change mine and add one more washer on each side.
You could use a larger nut and insert it between the frame instead of using washers.. ,or anything else that would add some height.