@MichałFita Most likely was originally a 69" radiator (old standard size). That would have been 1750mm plus 85mm for the valves would have given the 1835mm centres. When replaced they probably used an 1800mm rad, hence the zigzag. Using a 1700mm radiator with a pair of say 20mm radiator tail extensions would do the trick (1700 + 2 x (20 + 47.5) = 1835mm), or a 1700mm rad with some long telescopic valve tails.

Perhaps the same but with something designed to be outdoors - a felt roof shingle would likely work just as well as a foam block.

Out of curiosity, did you set both fans to max speed before connecting the controller? There is a note here which says: "Before installing your fan switch, please ensure the fan motor is set to high speed. If you notice that after you install your fan switch the speeds are not as fast as they were prior to installing your switch, it's likely that the motor was not on high (please adjust if you find this is the case)."

Self-drilling screws like this are also sometimes labelled as MDF screws - they help to make a hole through the fibreboard rather than pulling up the fibres into a big lump.

See the comments under the question - the OP stated the bolt was angled such that the head was not between the jaws and therefore not being measured.