You mention that you use the trolling motor sparingly. If you're not
deeply discharging the battery a small solar panel should work.

One of the batteries on my boat never gets a deep discharge. I use a 14
watt panel with it, and that panel keeps it up just fine. It was cheap
at Harbour Freight, about $40, and it's small enough where you don't
need a regulator. It measures about 12" x 18".

I tried one of the smaller ones, can't remember the wattage, but it
measured about 4" x 12", and it was not able to do the job.