Sharpening jig using a whetstone. Baseplate has guiding rails, knife just gets inserted either in the 25° slot or laid on the 20° flat of the slider. Not even quite sure what to use the 25° for besides pocket knives that get heavily abused anyway. Could probably use an 18° guide even.
Angles available: 20° and 25°
Stone dimensions: 150x50x25mm
Example stone, can be had cheaper - I got mine for $10
You'll have to fix the print orientation of course. Baseplate doesn't need any supports, the slider however most likely does.
I printed the slider on the "butt-end", 25° part being lower than the 20° one. That way I only had to remove support material from the 25° slot.
The rails of the baseplate are just long enough to fit on my Anet A8's 220x220 bed, by printing it diagonally on there, resulting in a boundary box of 210x210mm.