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I don't know how long you want it to be but my best advice is to make it in 2 pieces. Make the first piece the tall flat part and then make the second part of it the down flat part. this will make it way more manuverable and plus you could always set it up as a flat up or whatever. i made mine like this and once it was done even the 2 pieces were pretty heavy but two people were able to move them.
basically all you will need to make the box are a bunch of 2x4s, 2x6's and then 2x1s or 2x18s for the top part where you will mount the pvc of hdpe that you are going to use. what i did was i got 3 2x6's and 3 2x8's then i used the 2x8s on the top and mounted the 2x4s UNDERNEATH those pieces as the supports then, i used the 2x6's on the bottom of the frame and attached the 2x4s to the SIDES of the 2x6's. if you can try and get a 2x6 that will run the full length of the bottom of the frame so if your going to make a 18ft fdf try to get a 12 ft 2x6 so it will be more rigid for the down flat.
the angle where the down part meets the flat is crucial. make sure you lay eveything out at first and then make sure you have the angle that you want before you start to cut. you want to make sure that the angle is flush. i put 2x4s at every foot and a half or so and it is real rigid.
here are some pics of my box.... its done now and i will probs take some pics later this week when we finally get to hit it.