I have an almost new 150mm Thomson Covert dropper for my new build, BUT at full height my seat ends up about 20mm to high for my normal riding position.

Is there any way to reduce the travel internally with a spacer? Has anybody tinkered before?

I have used a paracord banjo externally before to limit travel on test rides but it is a bit of a naff solution.

can you not just insert the entire thing 20mm further into the frame?

In my experience of 2 different Thomson droppers just wait about 6 weeks and it won’t fully extend anyway, so you’ll lose at least 5mm of height there.