I bottle it in well labelled surplus 2l soda bottles and take it to the HHW site if there is another reason to visit there; it is a 20km round trip in a direction I don't head off to very often when they are open.
If there is not a reason to go and the fixer bottles are starting to pile up in the warmer months I evaporate it down to a crusty sludge. 4L takes about 3 weeks in the best sunny late sprong weather for me.
RA-4 Blix is the bigger hassle for me. It gets dehydrated too, mut is much ickyer to tidy up after.
zapped out toners get moderately dehudrated and handed in at HHW, or just handed in fully wet.