Cheap score, unwanted seat pan from someone else's build got it for next to nothing……...
Bates style, 1.6mm steel zinc plated seat pan. 9" wide x 13" long.
M8 threads, rear threads 6.25" apart.
There are some imperfections in the zinc plating, and a small amount of rust under the flat bar, otherwise a perfectly good seat ready for upholstery.
Came with foam for upholstery, density may not be appropriate. Might look at getting a cheap yoga mat or some other foam to replace it. Possibly layer up some different foams.
Cut a small piece of black rubber fuel line to edge the seat. Worked out great, will go buy a metre length of it to do the entire outer edge. Cost about $3.50/m