I have used the paviors that you lay on drives, they don't need a deep trench, not cost too much and you can lay them long ways or narrow ways. i just sumk mine onto sharp sand
Use lengths of treated wood screwed (easier than nailing if you have a drill) to stakes (preferably oak). Stakes 2" square by 18" or two feet long depending on your soil and slope. Your lengths of wood can be simply standard fencing rails, 87mm x 37mm, which you can get in 3.6m lengths from your local woodyard or farm supplier, or you can thicker and deeper sections if you like. Fix them so that theeir tops are flush or just above the soil surface. Excuse the mixed measurements!