Hi Sindarius, the purpose of the springs is not to avoid filament hitting the cover, but to help when MMU fails to load the filament into the extruder. The distance traveled by filament when loading is not very reliable, and sometimes it stops before the extruder gears and doesn't reach the inside, so the extruder prints "air". Using the springs and calibrating the loading distance a bit longer help preventing this. The springs avoid pushing the filament too hard into the extruder gears. When the filament reaches the extruder gears and MMU still feeds it, the springs absorb the extra distance.
I used two steel spring taken from standard ballpoint pens (4mm diameter). You could also buy one of this:https://www.aliexpress.com/item/dia-0-6mm-D-4mm-L-10-30mm-10pcs-stainless-steel-compression-spring-non-corrosive-tension/32909937531.htmlhttps://www.aliexpress.com/item/2pcs-Spring-steel-pressure-spring-Y-type-compression-spring-0-3-0-4-0-5-0/32965070428.html
A 4mm diameter should fit the screws.