Re: Confused about carbon seat sizes
I played around with designing a foam lumbar pad for the Euromesh more than several years ago, and had some prototypes made up from a curved profile template I provided. The pads worked pretty well, but I never was able to source an economical foam that held up. Mine seemed to last about a season or two, and then start to compress.
So now I use one of the prototypes, which I have added additional height by building up layers of perforated black foam shelf liner material. Much like building up a topographic model, each layer is a bit less than 1/8". I use spray glue to secure the layers. I have also added subtle torso padding with the same material for a perceptible cradling effect. This all goes under a standard Vent-i-sit pad. I then covered the whole thing with a Bacchetta seat cover, which does a great job providing a finished look, and I prefer the feel over the bare Vent-i-sit. The cover has velcro on the back to attach to the seat mesh.
However this submarine sandwich of pads is fairly heavy, but comfortable and supportive for me. You might try experimenting with some yoga pad material and shelf liner for smoothing edges. I found a bread knife works pretty well cutting foam. Could be a folded towel might work OK.