Your plot sounds ideal for a thick compost mulch, 4-6 inches if possible, with stones and roots left in place. Old roots will gradually rot, and help with drainage as they do and feed soil fungi. Stones often contain minerals and help with drainage too so they are not all bad, except for being awkward to work around when at the surface, so covering them with organic matter is good. I have a lot of stones which I never see, whereas the ploughed field above (same soil) has a lot of them on the surface.
Bed sides are not a necessary part of raised beds, unless you particularly want defined and tidy edges. You could cover the whole area to create a bed of 1.5m wide and 4m long, and you can tread on it to reach the middle if need be. Good luck.