The Nippy inflatable seat bag is covered in Gould’s book and pictures are elsewhere on this site. The inflatable bags degrade and 2016 prices were over £120 per seat bladder.
Many Nippys have been ‘Rubified’ over the years due to lack of available original parts. Seat bases from Rubies and Box saloons are present on Nippies and we cannot wholly disprove that later cars could have had stock items introduced into the Nippy specification, just like its availability with the standard Ruby engine.
The disadvantage with the later seat bases having an obvious sculpted form under any cushioning is that they make driver and passenger sit higher in the car – for taller people, possibly not through the windscreen, but over it.
It is possible to use what you have to convert to Nippy specification, if there is enough flexibility in the cover material. You can see above, restored seats that have a 5″ height at the front edge. When uncovered, these wooden Box seat bases were found, shown below:
Other later Ruby bases have similar shape from a steel ‘bucket’ – you can see the bag flap below:
The remaining edges of the seat squab leather were trimmed to allows sides of 3.5 inches, and tent canvas sewed in underneath to create the bag with opening at the rear:
There is a recess in a Ruby seat frame so a 1/4” ply fits in nicely and keeps you off the floor but as low as possible (in the pic where it just looks like the floor…)
For reproducing a bag, a “Fatty 4.0-4.9” mountain bike tyre’s inner tube, secured as per the diagram in Gould’s book with a first retaining band using an 1” section of Austin inner tube (so it stretches), and the second a flat band glued to enable the other gathering of the tubes. These were less than £15 delivered for a pair.
This picture shows an intial comparison of ziptied 17″ Austin inner tube (left; not long enough for the area of seat required) next to the Fatty MTB tube (right) with a bit of a size 9.5 shoe for scale:
I was thinking it would need some 1” rubber underneath but this felt pretty good to me, after seeing and feeling Chris’s Gould original seat bag. The inflation required is very minimal; just enough the keep your bottom off the deck…
To deal with the seat hinge bolts we use roofing bolts and stuck rubber over them to avoid having to have another layer in there. Rubber patches were also stuck over the splayed rivet ends from the upholstery.
Harry Fox has been working on exactly the same ideas, including the trial of some double ended inner tubes. He has adopted a plan of making the seat baseplate – protecting the bag from the floorpan and the bolts attaching seat frame to the runners – integral to the seat bag, by bonding the seat cover material to a 16 gauge alloy sheet (below) much the same in appearance to the steel baseplate seen elsewhere.
Both routes give a method of getting you several inches lower into the car, for a bit of time and not much money.