I've got a car screenwash tank with inbuilt pump. I planned to run it at 6V or so as it's not meant to run continuously, and running at 12V I think the flow will be way more than needed.

Only problem is how to attach a return connector to the top, and making a wooden crade for it. If it proves too difficult (maybe it's polypropylene which won't glue) I might try using a small car header tank and I've got a small brushless pump which will be fine for continuous duty.