Used 2 - 2 X 12s X 16 ft & some old plywood for the ends.
Got a load of rejects from Lowes last year cheap, use them for soil boxes.
Put it in the garden & filled with fresh compost. 5 wheel barrow loads of a small pallet bin that was done allot better than I thought. Topped the compost with 1 wheel barrow load of Horse Manure compost. then shoveled in a few inches of garden soil. Mixed with a pitch fork. watered. Letting it set for a week until planting time. It will be Granddaughter's garden spot for peas.
1st pic full of compost from pallet bin, 2nd pic mixed & watered.