The first set of springs I made was a bit too heavy, so I picked up some finer strip. Using 0.75x0.25mm allows the correct four leaves, and looks reasonably close to scale:
They are less wonky than they look in the picture, honest - I think it's because the picture is much too wide-angle!
These are really easy to make - if you cut one length of Evergreen strip (as above) into four, you can stack the pieces and build four complete springs flat on the workbench. The end brackets are 1mm slices of 2mm U channel, and the middle one is a 1.5mm slice of 2.5mm channel. These dimensions are eyeballed from prototype photos - I can't cut any more accurately than that anyway I formed the curve freehand, the end brackets hold it in place as soon as the glue grabs.
To show off the individual leaves, I scraped down a bit of leftover strip to make it thinner, and cut it into tiny slices. Two of these are popped between the leaves (insert a scalpel point and twist to open) before fitting the centre bracket. Fiddly but satisfying. The combination of these and the shallow curve makes the wagon look like it's running loaded, I hope. One problem I had was while mounting the springs on the underframe, adjustments to position tended to compress the leaves and make them look a bit concave. I think gluing in the centre only (and to the spring hangers) might be a solution for this, I'll try that next time.
The hard bit is still paring off the old mouldings. I managed to do four while watching (or at least listening to) a Bond film, but I did stab myself twice in the process. More practice needed, but at least I have enough underframes now for the remaining MKAs