there are several ways of extracting silver ions from your spent fixer, sunlight is not one of them ...
pass electricity through it ( electrolytic ) and it will plate out on a cathode but leave a bunch of silver in your fixer still
plate the silver out using a reactive metal, but then you have a bath full of another metal to figure out what to do with THAT.
you also might have to worry about PH of your bleach-fix to get these systems to work to your liking.
evaporation might be useful, but then you have all that sludgeDUST..
do you have a mini lab/school near you? household chemical disposal day through your public works department ?
it might be as ez or easier to bring it to them ...