This is my mock up of planned compression / drawing, will have 18 x springs in total ( 3S6P).
So far has cost £10.77 for the springs
https://www.ebay.co.uk/itm/Compress...a-e7d6-46cc-9b0e-f90b5ac460ca&redirect=mobile + couple of £ for the 12mm OD plastic pipe(sleeve on inside of spring to allow threaded bar to slide, plus on threaded bar along it's lenght to ensure isolation against cells) - all other bits are sourced from work for free? - hence why 8mm threaded bar - 6mm would be ok but none at work ?.
Spring rates chart is as tested under various compressed lengths, also compressed spring for a week to check for long term deformation and seems good.
So as far costs go- negligible, just a bit of time in man cave.
View attachment 113409
View attachment 113410
Note - this is my take on a solution for compliance with requirment, you make your own minds up how you wish to build.