Hello, sorry for delay,
SS-PG is a handmade modification of a Ssquare, puzzle was cutted by hand, afterwards cut pieces were glued and cut again,
finally puzzle was covered with black sheets of polystyrene with a thickness of 1 mm, illustrations of the various steps are in the "building"section.
the most obnoxious was to cut the adhesives of the top and botton.
to create it, it took about 20 hours,but, at least it came out a perfectly symmetrical puzzle and it runs fine.