Making a simple planter box that can survive wet weather

I want to make a small outdoor planter from inexpensive boards. It will sit on paving, get regular rain and be watered frequently, so I'm more worried about drainage and rot than making it look fancy. I'm planning gaps along the base, raised feet and a lining that doesn't trap water against the wood. Is there a practical finish or wood choice that meaningfully extends its life without costing more than a ready-made box?