Actually, you do not have to straighten the islands out by hand, the command Mesh > UV Unwrap > Follow Active Quads is perfect for quad strips like these.
Basically, follow all the previous steps:
- Remove Doubles, so your mesh is contiguous again (it will merge vertices together that are in the same position, you can also enter a tolerance value that will then get points that may be a little off, too)
Mesh > Vertices > Remove Doubles
- Mark your Seams with Mesh > Edges > Mark Seam
around the lid and in the fold front and back should do fine
- Go to Top View, Select the lid by hovering the mouse over it and pressing L, then, in the Operator Panel, select Delimit by Seam. This will keep your Face Selection limited to the area you just defined.
- Since it is flat, you can unwrap it easily with Mesh > UV Unwrap > Project from View. If you want to avoid clutter, you can now go to the UV/Image Editor and move your unwrapped island off the 0 to 1 Square. This is just to keep the other islands from landing on top of them when you unwrap them next, so skip it if you want.
- Select one of the strip parts by hovering over it and pressing L. This will select Linked Face. Put the Delimiter in the Operator Panel on "Seams", just like before.
- To make sure the Unwrap works properly, I recommend first resetting the unwrap first with Mesh > UV Unwrap > Reset
- Then, first deselect one of the faces on the island you want to unwrap and immediately re-select it. You will notice, its color is different from the rest of the selection. This is because this face is now the Active Face.
- Now unwrap the strip with Mesh > UV Unwrap > Follow Active Quads
- Usually, Length Average gives the best results on strips, but you can toggle through the option to see if you can get a better fit.
- You should now have a perfectly rectangular strip of quads in your UV Map. It may require a little scaling along the X- or Y-Axes, but it will make texturing very easy.
Side Note here: one of your strips has a hole in it that is made up of N-Gons. This does not go over well with Follow Active Quad. I would suggest you model that shape with quads. Otherwise, you will have to stitch those faces in by hand or straighten all those UVs by hand.
To ease that pain, take a look at the UV Squares-plugin. Just search it in the Plugin List in Blender, I think it is part of the trunk.