Bought all this from Home Depot. Couldn't find a stainless bolt with a shoulder in 40mm length. The clip is a 1/4" hitch clip. Started with a M6x40mm bolt, cut off the hex head, flattened two of the sides, drilled a 3/32" hole and secured the pin in the seat with blue loc-tite.
Attachment 1096
Attachment 1093
Attachment 1094
Attachment 1097
Attachment 1095
It take a little to get the pin lined up to go through the hole. I also test fitted it with the bolt screwed all the way to the shoulder and just let the pin hold the seat without having it stick through the bottom, it works as well, but leaves an empty hole showing underneath.