So here the rather ugly almost universal headband pad I made...
It's definitely more work than LFF's suspension headband or any glue-on padding previously discussed, not as nice looking (could be if you put more work into it than me!), but fits on many headphones. Works on all headphones I own in any case.
It all starts with this:

Note that the red and black measurement is from end to end (of the red and black line respectively), not from center.
You will need one sheet of this, plus one sheet of each half separated by the red line. I used thick pleather as it was what I had extra, and I also find it very comfortable.
Next, cut a full sheet of the previous shape of foam. I used a sheet of 6~7mm open cell foam.
Step 1: Sow the top and bottom together on the outer edge; the flat sides should not be stitched together. It should look like this (minus the foam).

Step 2: Stitch the foam onto the bottom piece, on the inside. Leaving an edge of ~1cm around the edges is fine, all that is required is the contour.

If desired, a band on the inside can also be stitched to give more of a two-point support feel.
At this point it's already essentially done. It may not hold very well to every headphone though, so additional steps may be taken to secure it.
Whatever you call these clips, but I used two of these, and two strips of pleather. Forgive my awful sowing skills here. The results are as shown.


Different clip on designs can be used to make things look nicer. In the pictures are prototype 1 and 2.
Just to give an idea of how it fits on other headphones... the Q701, which was what drove me to make one. The grooves under the headband just kills me, I don't know how people feel comfortable under those.

The clips are spaced so that you can still see UINCY JONES. Sorry, Q.
The sides are also cut out to not obstruct the L/R buttons.
Here's a vintage headphone, The Headphone People Headgear 2001. Little to no padding under the original pad...

Fits on the PortaPro too! As you can expect, the DIY band is much wider than the stainless steel band, but it still holds quite well. Solid when worn as well.

And finally, Fostex T50RP. The edges are a on the small side. Adds padding to the rubber basically.

So it doesn't look all that nice. Prototype 1 (the one that is completed) pleats on the interior, but you don't feel it on the head.
The idea is something that will fit just about any headphone, easily removable, and not too terribly hard to make.
Probably goes better in the DIY section but eh... hope some of ya will find use for it lol. I certainly did :P