First things first, have you ever made any type of net before? If you have, a longnet is quite simple, knit a row of meshes to the desired width ( I like 14 meshes at 4.25" mesh), then carry on till you get to your desired length.
For the bow back box, when I made one, I used 1/2" ply for the main body of the box, and then used thin ply (can't remember exactly how thin, 3 or 5 mm I think) for the bow part.
I'm sure somewhere on her there is instruction videos for net making and there is also bound to be sizes for a box on here.
Cheers Luke