built the box as a 8' x 4' rectangle. I bought some landscape spikes and predrilled the holes for those then pounded them together. They aren't super "raised" but enough that it totally keeps out the weeds and I just buy a couple of bags of soil and peat moss to replenish as needed. I've had them for 15 years and they have held up well. One board is $17 so it takes 3 boards (two 8ft sides with two 4 foot sides). I think when I did this years ago the board was about $8.

https://www.lowes.com/pd/4-in-x-6-in-x-8-ft-Douglas-Fir-Lumber-Common-3-562-in-x-5-625-in-x-8-ft-Actual/1000028917