Fiddle block with cam cleat
I have tested the block with about 70kg static load (me) plus some jumping up and down so probably about 100kg total with no signs of anything about to give up. This was printed in PLA with 6 perimeters and 30% infill. I obviously don't recommend using this for anything remotely safety-related.
The cams are not spring loaded, which means they are quite picky with the rope diameter. These work fine with 3-4mm rope.
Use M6x40mm screws for assembly, there are no standard size that exactly fits the shorter ones so I just used 40mm for everything and cut the shorter ones with a hacksaw after assembling. Nyloc nuts are great for this.
I intend to use this as a kicker on my small sailing dinghy (terhisail 375), we'll see how that works out...
Here is the complementing fiddle block without the cleat: https://www.thingiverse.com/thing:3691483
EDIT: After 4 hours with good winds and one hour with too much wind I can say the kicker works very well.