..........Great - the "flanges" as I call them, not knowing their proper name - are the plastic bits that slide into the webbing straps on the tubes - I am pretty sure these are all the same size and it will just be the seat length that differs to fit various beams.
I have not see any other way of attaching a seat to a tube - I have attached pic of a 360s which shows the two seating positions - whilst inflating the tubes the seats are attached into the straps and as the tubes inflate they become fixed tightly in position.
Are your seats this style - if so great as all I will do is take off the positioning pieces and afix them to the correct length seat.
If so - we just need to agree a price and arrange collection / despatch.
Buy it & Use it, then sell it and buy something bigger