Hello all , I currently have a 1.8 s mx with cloth seats, they are the removable headrest type unlike some with the oval built in head rests , they are comfortable but after a 100 miles or so the seats feel a little flat and seem to lose support they don’t go flat but feel like they could do with more support especially at the angle between the backrest and seat base , anyone know of a mod or recommend a replacement type to improve comfort ? 
the best seats i’ve found are the ones from the mk2.5 the high backed ones! but any seat from a mk1, 2 or 2.5 will fit…
My seat had lost its cushioning and I suffered from a numb bum, so off it went to a trimmer for major surgery, I now have a comfy seat.
The factory optional “Driver” seats I have are super comfortable and very supportive.
But are a little rare.
An owner in the Phillipines (which had cars with the same Euro-spec seats) had his seats rebuilt, repadded and recovered to an incredible standard:
http://clubroadster.net/vb_forum/showpost.php?p=1082&postcount=1
Although later seats can and are fitted to earlier cars, the seat height can change; generally the seating position in Mk2/2.5s is higher than in the Mk1.
Over time, the seat squab foam breaks down; the foam is actually moulded aroun wire springs. As the foam breaks down, the seat becomes a bit flat, and sometimes you can feel the spring wire. This seat base, which will be a similart shape to the UK seat, has just started to fail (wire can be seen pulling through).
Thats how it feels I can almost feel the seat rail just under my coccyx so I would have thought a small triangular cushion may give some support until new seats are sourced thanks for the pics very helpful. 
The foam bases might still be available from Mazda; IL Motorsport used to do the early Mk1 MX5 bases for about £100. Proper replacement foam bases with the moulded in wires might become invaluable in the future (heavy hint to the aftermarket).
i have had both a mk1 and now a mk2.5 and the 2.5 seats are much more comfortable get them