I've fitted Wilbers progressive springs to my old trusty Tzr250 1KT bike in the past. The instructions asked for no preload spacers but this gave back a soft & sluggish response from the front suspension even with some washers added as a preload.
So, I decided to make my own preload spacers as it is impossible to fit the stock ones with the new springs. I draw a couple paper templates of a rectangle that measures about 96mm x 30mm. Then stick the paper to 1mm mild steel sheet and cut with tin snips.
You should end up with something similar to the photo...
So, we now have a couple mild steel rectangles that we need to form as cylinders.
I used my handmade slip roll to form the steel templates but a rubber mallet and a suitable pipe used as a mandrel will do the job as well... you do not have to weld the cylinders, just trim them a bit so they are true and exactly the same dimensions to each other.
The actual construction will look like this... ready to fit in the front forks.
Gladly, the front suspension now works as it should and the external preload adjusters have the effect I need for setting up the bike.
I hope you found the info useful. Cheers!