Great that you have dark soil and because there are no weeds, a two inch mulch of compost (i.e. well rotted manure) is fine.
I wonder if you need wooden sides, if it was me I would simply make beds with the thin dressing of compost, though that will result in muddy pathways. So you could spade a couple of inches of soil from 18 inch pathways onto what become slightly raised, 4 foot wide beds, then add compost on top of them and some less decomposed organic matter for a path mulch.
