Literally anything will work.
I maintained two big marine batteries 20 yrs ago with one 10 watt panel. It was wired straight into the battery giving the full 18volts. The batteries lived a long long life.
Better to have a charge controller.
Small panels work, but for a few extra bucks as long as you are doing all this install work anyway it makes much more sense to get a bigger panel.
My strong suggestion is to get an ebay 100 watt mono crystaline glass panel that ship for about $80. Get a cheap MPPT controller that is about 20 amps for around $50 (not the $19 one), and wire it with 12 or 14 gauge wire at least.
This will both charge, maintain your battery and also give you extra DC power when camping.
If you do any off-grid or boondocking camping, then get at least 200 watts of solar and a 2nd battery. You will be glad you did. Though it is bad to pair a new battery with an old battery if you run them parallel. Serious boondockers get two 6-volt batts and run in series.
