Making cross braces and assembling carcass:
Place one of the sides on the worktable, inside up. Remember the mark on the side for the end of the kerfing? Now we'll mark and cut it to allow for the cross braces. The four main cross braces are milled to 1⅛" x 1". Measure accurately the actual width of the flashing and add 1/16” - 3/32” for the length of the braces. There is paint thickness to consider and you don't want to face trying to trim a sliver from the edge of a 6-foot length of flashing, so give a little leeway. A snug fit, rather than tight is what we're shooting for for the top. Cut the four braces at the same time with a block clamped to the cross cut guide on the table saw to ensure they are exactly the same length, with perfectly square ends. The square ends are important because they will help you to get the carcass together accurately so the bending ply and aluminum will go around with a minimum of fuss.
Measure back along the kerfing from the starting points for the ends, both front and rear, to allow for the braces width and cut the kerfing completely through (but only the kerfing) carefully with a dovetail or small back saw. Use a small hammer and block of wood to pop the short piece of kerf off and clean the surface with a chisel. Choose two points on the top of the side, roughly spaced across from the bottom braces. You basically want four supports spread somewhat evenly. The placement of these two top braces is somewhat arbitrary. The four braces are to support the two sides parallel to each other, with the sides lined up. The bottom braces spread the sides exactly and also provide an attachment for the ends of the bending plywood (which will be under some pressure) and the locations of which are critical, while the top two braces merely spread the sides exactly. It may be easier to locate these top two by first choosing spots on one side, marking the width of the braces and cutting the kerf and removing those sections, then place the sides together and line everything up, and mark the uncut kerf using the cut kerf on the other side as a location guide. The outer edge of the braces line up with the edge of the kerfing so there is a continuous gluing surface all around when the braces are in place.
Set up the table saw to cut the bending plywood. Set it up slightly under the width you cut the braces, again to allow some leeway, say 1/16". Cut two strips of bending plywood. Lay a side on the work table, overhanging enough to reach to the bottom braces, inside facing up. Place a brace in position and nail in place. I use a brad air nailer. I suppose it's possible to hold the brace in place with one hand while using a hammer in the other and a nail in the third hand to do this, but an air nailer will be most accurate and easy to single hand. Think of it as an excuse to run out and buy one if you haven't already...oh, AND a compressor too, you'll probably find the nailer works much better with a compressor. I am used to working alone on complex projects and figuring out ways to replace a helper with inanimate objects clamped together, so my approach is as a lone woodworker in the wilderness. If you have assistance, more power to you.
Attach the rest of the braces in the slots you've cut in the kerfing, one at a time, lining up the edges carefully. Place the other side on top of the braces and locate and attach each one carefully - now you have the skeleton of a carcass.