I made a box with 2 x 6 wood. screwed slats to the bottom for drainage. It is 26" wide by 52" long and 20' deep.
I used some wire mesh on top of the slats.
I attached 1x6 pcs of wood along the 2 short sides to attach the heavy duty castors to in each corner.
lined the bottom with a pc of left over trampoline material. Lets water through but very heavy duty and ridged. Stapled it in place.
then I lined the whole sides of the box with the weed block material. Stapled it along the top edge and then cut off the excess.
framed the top edge with 1x4s to hide the staples and look tidier.
I made a 2x2 frame to put on the top and drilled holes half way in (to fit the pipe) and then bent pvc pipes that I had left over from another project to make an arch. I covered the arch with heavy plastic in the spring to make it like a mimi greenhouse. Used large binder clips to hold it in place so it is easy to open it up by folding the plastic back in the day and covering it back up at night. the plastic comes off in the summer. But the pvc pipe is useful as supports for the tomato plant that grew quick well. I also planted Basel and onions that did very well.