The ultimate purpose of a lens hood is to prevent unwanted light from impinging on the front element of your lens and it doesn't have to look pretty or cost a lot to achieve that goal.
It helps if the hood's cheap, light weight,
folds to take up almost no room in your kit bag.and is somewhat adjustable in length.
Here's a step-by-step description of making one such hood -- no math required. This one's made from a file folder to show the layout and it can't get much cheaper. This took all of about 30 minutes to complete including the decision to do it and processing the photos.
1. Materials: use your own judgment as to what will work and how much you want to spend. Heavy-weight file folders are a good place to start but I've made 'em from poster board as thick as that used for the back of writing tablets. Start with something dark or spray paint the end product with flat black paint for best results.
2. Layout: there's no particular advantage to using other than a square shade which avoids having to design origami-type fold lines. As you can see, unequal sides simulate the tulip-shaped hoods and can be custom trimmed to any degree of precision you wish. The width of the "box" should equal the diameter of the lens barrel at the point of attachment. You can make it a tapered cone but that just complicates things and necessitates that origami trick to fold it. A good starting length is also equal to the diameter of the lens and this can be "trimmed" by moving the hood forward or back on the lens.
A filter ring MAY be a good aid in laying out the dimensions but be sure it's the same diameter as the lens barrel.
A smaller hood might be laid out so as to have only one glue seam if the material is large enough. I suppose you could go for an octagon pattern rather than square if you wanted to get closer to a round hood but still have it easy to fold.
Why not just trace the outline of one of the tulip hoods? Well, you could do that, but remember, a primary goal is to have a collapsible hood and round or tapered designs present fold problems.
3. Assembly: Glue, tape, rubber bands -- I assume you'll sort that out in the layout. Note the fit of this pictured hood is a little "sloppy". Two reasons: there's a ridge on the lens barrel that adds about 3mm to the diameter ahead of the rubber band, and the mounting tabs are just a little wider than necessary - they're clipped in the corners here but could be trimmed to a better fit as well.
Even if you already have a hood for your lens(es) you may want to make one like this as a space and weight saving solution for the trail. This hood's 120mm inside diameter! Which one of these would you rather carry?
I don't even want to
think about the priceof a replacement hood for the Tamron SP 300/2.8 shown here even if I could find one. But more inportantly, this demo model won't go to waste; it'll be painted and tucked in the kit bag for a 'sunny day' afield.
(As a side note, here's a mod I've made to my tripod legs over the years. One fixed brace is disconnected and replaced with light-weight chain and a hook so that one leg can be adjusted independently. Works quite well in tight spaces or against a wall and the brace can be reattached if desired.)
Did ya notice I can twist that hood around the barrel to catch a stray light source and slide it back an' forth to adjust the length and vignetting? Even make it oversized or cut it up or bend a corner to compensate for the lack of a perfect tulip pattern while on site.
Oh, yeah, the metal Tamron hood weighs 3/4
pound where as the cardboard one comes in at 3/4
ounce (with the rubber band)!. An' the tabs are long enough it'll fit the SP 180 and SP 80-200 as well.
I'm a lifelong fan of pragmatic, field expedient solutions
that I can afford an' I'd have to take out a mortgage if I had to replace the hood for that SP 300/2.8.
Besides, it's fun to watch the guys with $$$$'s-worth of gear go dumpster-divin' after the ones you throw away when they don't have anything with 'em in the field.
H2