Arguably my favorite subject.
I've made a bunch.
Being honest, I work several jobs and am an expert procrastinator - it doesn't really work out to try to sell these, or really do any "extra-curricular" business.
Cloning and scaling the "Merry Muscles" seat design has been done. I have patterns and crude instructions (my notes) available if you would like.
The basic idea is to get one of the baby ones, measure it, draw that shape in the computer somehow (I use EAGLE, an electronics design program, but I've also managed to do it in excel (yes, the spreadsheet)... anyway, get it in the computer, print it out "scaled up" (150% seems to be good for getting most infant/toddler products up to a small-adult size)... pin it to fabric, stitch, remove paper, you've now got the seat. Add whatever hardware is needed (a metal ring [I used a bicycle rim with the spokes removed] and some rope for the Merry Muscles design, most other designs will require a couple of "sticks' for the seat ends) and jump to your heart's content.
A brief word about scaling: if you start with a "baby" product expected to fit a 40-pound toddler, and scale all of its dimensions up 150%... the amount you're actually scaling it is 150% of 150% of 150%. Remember that you've enlarged it in all three dimensions. It's actually a scaling of 337.5%, and a toddler-shaped person that size would be about 135 lbs. If you were thinking you needed a bigger scale, you're likely wrong. Scaling it only 2x would make that 40-pound product fit up to 320 pounds, which is clearly absurd.
Going a step further with that, remember that a product whose manufacturer says "40 lbs" needs to fit a short, wide 40-pound toddler as well as a taller, slender one. Its seat opening is going to be a bit bigger than needed for an "average" 40-pounder in order to accomodate the wider ones, and the top-to-bottom dimension will be a bit taller than needed in order to accomodate the taller ones. This overshoot is going to be carried through your scaling. 150% scale is absolutely going to be suitable for an "average" body up to 160 lbs or so. I didn't put any math into this estimate, it's just my experience from shoving various victi- er... friends... into mine.