Well, ok... it's *one* way to build a C box
Step 1: find a level area that you can chalk some lines.
2: Pick a radius. I went with 11' from the outside edge of the box to the center point
3: From the the center point draw your outside curve on the ground using a string. Have someone or something hold the sting at the center point while you move the end of the string and chalk in the path of the arc. See the purple chalk on the ground in the pics below. Draw this arc longer than the length of the box you think you want then come back and mark the actual length using a cloth tape measure so you can follow the curve. I went with 8' so that my plywood would fit in one piece.
4: Draw an inner arc so that you can see the width of your box on the ground. I went with 16".
5: Wood. Here's the run down for what I used...
5 - 8'x2"x4" treated
several - 3" and 2" exterior screws
1 - tube of wood glue (I like to screw and glue construction like this)
3 - 1/4" plywood 4'x8'
6: Build the ribs. See first pic below. Here the bottom board is 16", the top is 18", the front is 17" (flat to tip), the back is 25" (flat to tip). The angle of the top board turns out to be about 25degrees. I will not be sure if this is a good angle for the curvature of a 11' radius box or not till this winter but it feels/looks about right.
I've spaced the ribs out equally so that they're about 2' apart. You'll want to draw a straight line on the ground from the outside arc to the radius center point for each rib so that you'll have a guide to show you how to align them. If the ribs are not pointing to a common center point you'll really struggle getting the top to fit right.
7: Add bracing between the ribs. The angles get a bit tricky here. Measure twice cut once! If you have the ribs aligned correctly the angled cuts will be nearly the same. I could have used a similar bracing on the front as I did on the back, but I opted to use some window trim I had lying around on the bottom. Looks pretty janky really. In hind-sight I'd brace the front near the bottom of the ribs using 2x4s. Keep in mind that the box doesn't need to be fully rigid yet. The outside plywood covering will give it much of the rigidity you'll need.
8: Cut the plywood to match the back and front heights. I got lucky here and was able to use one piece of 4x8 plywood to cover both. Don't cut the ends till you get it attached to the ribs. You'll need a second person for this bit to push the plywood in place. Start in the center and move outward, screwing and glueing the whole time. Definitely use glue here, by the way, as you don't want the plywood pulling through the screws over time. Keep in mind the the plywood is structural in this design. (I was hoping to keep the weight down).
A shot from the back. My 8' long plywood was not quite long enough to make it from end rib to end rib so I had to sister in some cheaters on the end ribs to give the plywood something to attach to. I'm no master craftsman :P
9: Once you've got the plywood attached to both sides and the ends you can attach the top pieces. Yep, I used two on the top for added rigidity. I tried one and it just didn't feel right. Could have tried a thicker piece of plywood on top, but it may have been very difficult to get it to conform to the funnel section shape of the c box. So, again... using two people and a quick clamp or two. place the top piece of plywood on the box making sure that it is conforming to the curvature. Zap a couple of temp screws into it to hold it in place then crawl underneath and trace the outline of the shape you'll need to cut out. Alternatively you could just attach it completely to the box and router/jigsaw around the box... your call. I traced and then used my circular saw on a flat table. I screwed and glued two 1/4" plywood sections to the top.
10: Now for the plastic. HDPE is the material of choice. My 4x8 sheet of 1/4" HDPE cost $80. Not bad. The cool part is that I was able to save a 8'x12" section for another straight box to double the length my existing straight box setup
Again, using two people push the HDPE into place, temporarily attach, outline and saw. (Again, you could attach first and router.) Remember to start from the center and work your way out as you attach. No glue here. Just screws. You'll need to pre-drill and countersink.
*note: this is not mine, i found this on snowgins.com, credits to: joyandrob