How to build a viewing hood.

1) get yourself the right material.

Black 0,12 mm thin opaque cloth. BK5 will do. Not 100 percent lightproof, but almost. Quite useable for a viewing hood.
0,33 mm black cardboard and a white marker.
Spray Glue for cloth, plastic, leather, foam and card board.
Box cutter, cutting tool, utility knife.

2) The drawing

The most important thing is to make a good drawing. If you still have the old but worn hood, you are lucky because you won't need to design the hood yourself. On the other hand, designing would give you the best possible glimpse how to fold it down in such a way that it can easily be set strait up and stay that way.
When copying the old one, the easiest thing is to do is to find the fixation of the two halves of the chimney shaped hood. Get it lose, lay it flat on the bench with the inside up and study the inside cloth or leatherette. You may see the outline of the stiffeners which are positioned in between the inner cloth and the outer leatherette. 
Carefully separate the inside cloth from the outside and you'll notice pieces of odd shaped plastic or cardboard. These are the stiffeners to keep the hood straight up and the free space between them makes folding possible. If the stiffeners are in good condition you may use them in your new hood. If so, you better mark their position. Take a photo. This helps you to reconstruct the new hood. Now take the measures of the old viewing hood and mark the stiffeners. For instance Left 1,2,3 etc. Right 1,2,3 etc. One by one lay the numbered stiffeners on their exact same position on the new paper copy. According to the condition of the stiffeners you might have to make new ones. I prefer thin cardboard. Open the camera cover and measure the inside viewing hood compartment on top of the ground glass. let's presume this is 12 x 14 cm. 
The cover with the carrying strap is supposed to hold the back of the viewing hood by its two brackets. Most of the time the length or height of the viewing hood chimney is about 6 cm higher than the length of the cover which is 14 cm as we have seen. So the height of the viewing hood should be 20 cm. Draw the rectangle of the inside hood compartment and subtract a few millimetres for play on each side.
Draw the cover side of the hood and divide it into two parts. The lowest part should be as long as the length of the ground glass compartment which is 14 cm. The upper part should be 6 cm. Draw the rear side which is 20 cm high and divide it in three parts. The lowest one would be half the  the length of the compartment which is 7 cm. The next part above it would be 7 cm as well while the upper with the nose recess would be 6 cm.

Depending on the type of viewing hood it should be somehow like the drawing above.
This viewing hood type is hold strait up by the camera cover. This explains the steel bar on the back, respectively on the front of the hood.
Self erecting hoods are different. Some suggestions below show how to make the right drawing.
Test your design by making a paper dummy.

 
The best you can get is the original hood including the stiffeners.