I needed a custom shock absorber for my RC car, so I made one.
For the 20 mm part you will need a 35-40 mm compression spring and for the 30 mm part a 45-50 mm compression spring with a 0,8 mm wire diameter.
The guiding rod is a hex socket cylinder head screw M3 https://www.ebay.de/itm/162035060113?var=461004073200

I recommend that you use an M3 insert, a 35mm screw for the 20mm cylinder and a 45 mm screw for the 25/30 mm cylinders.

When done with assembly superglue the cylinder cap on the cylinder.

Modified shock cap to fit an M3 brass insert https://www.ebay.de/itm/303614023248?var=602850729491

Added a slim version of the cylinder and shock cap (for use with a ball point pen spring)

Assembled slim version should have ~500g resistance

print at 100% infill, 0.1 resolution

Have fun ;)