In the search for cheaper springs I experimented with an Indespension leaf spring - mainly because they are about the right size and reasonably priced. The cost has gone up now to £38 but still not crazily expensive and discounts might be available.
https://indespension.co.uk/1200kg-7-leaf-spring-eye-to-eyeI dismantled the spring to get the long leaf from the pack. It's about 585mm (23") eye to eye, 50mm wide and approx 6.85mm thick. The camber from the eye down to the outside of the spring is 78mm.
Deflection with 83.2kg (183lbs) was 14.5mm (would be higher if no friction between eyes and workbench). That’s a spring rate of 320 lbs/inch
I cut 9mm off each side with a 1mm cutting disc. I burned through 4 cutting discs (£3.50) and it took less than an hour. By doing long cuts rather than concentrating in one spot, the spring didn’t get much above warm to the touch so the heat treatment won't have been changed.
Deflection at 32mm wide = 24mm = Spring rate 194 lbs/inch
Switching to metric
CK + driver approx 180kg
say 35:65 weight distribution gives 63kg on the front.
As a transverse spring this would deflect about 18mm at rest
Or 9mm as a pair of longitudinal springs.
The eyes are a bit oversize (visually).
I hope we might be able to test the spring out on a cyclekart.