The first thing I learned doing this was
don't use Krazy Glue.
This is a rough draft, I will refine it later when I'm not at work, and include pics;
I took pretty detailed pictures during the course of my repair experiments, including a lot of foibles along the way. At first, I tried super glue/krazy glue - totally ineffective. Snapped immediately, and left a crust that meant the crack no longer fit together. I worked my way up to a plan I mentioned a couple posts up, which was to use modeling sheet aluminum, and I used Gorilla Glue along the top of the band to adhere it, and then painted it - the result was very unsatisfactory. Lumpy, ugly, bulky.
Here's what finally worked for me, and the run up to it;
In order to remove the Gorilla Glue, Super glue/krazy glue residue, and spray paint I had applied over three repair attempts,I soaked the headband in acetone for a couple hours. This completely removed my prior gluing attempts, and also the paint - which will be a non issue by the end of this plan. It may have been helpful, from an adhesion perspective, that it was stripped down to the base material.
I purchased a couple sheets of this;
TowerHobbies.com | K&S Aluminum Sheet 4x10" .032" (6)
From my local hobby shop - less than $2 a sheet. You should only need one, but if you cut wrong, you'll wish you bought two. It was in the high end model airplane section.
You'll need fairly bulky scissors to cut the aluminum, and ones you don't mind ruining by cutting and a file to trim down any rough or uneven edges after you're done.
You will also need strong double sided tape, black electrical tape, and a Beyerdynamic DT-770 Pro headband pad. Beyerdynamic Part# 990.681, which can be purchased from their authorized USA service center,
Sound Surgeon Long Island. The headband pad is less than $10. It took them a couple days to get back to the message I left, but they definitely had them in stock, and its the only place in the Americas I found that sold it.
I cut two strips from the aluminum;
The first one should be exactly the same size as backing on the foam headband piece from the inside track. This is easy enough to do - lay it down on the aluminum, trace, and then cut.
The second piece should be the length of the top of the headband, from inside the angled bend at each end, and the same width. I used a piece of string to get the length measurement.
The inner aluminum strip should fit along the inside channel, where the headband pad was. Slide it in very carefully, it will be a tight fit and pushing too hard will crack your headband. You may need to take a pair of needle nose pliers/leatherman and pull it through the last 1/3 of the way.
This is the majority of the tension/support in this fix, and on mine where I didn't even glue the crack shut, the result is more sturdy than it was originally.
Bend the outer strip into a curve, slightly more acute than the curve of the headband itself.
Cut two pieces of double sided tape, their length the same as the width of the outer aluminum strip. Put them on each end, and use them to anchor the outer aluminum strip along the top of the headband.
Put the actual phones on their metal clip strips back into the headband, and replace the "Made In Ireland" plastic clips that keep them from falling out.
Wrap the entire headband, end to end, spiral, in black electrical tape. Be sure to pull on the tape as you wrap it to take advantage of the vinyl's elasticity.
The last step is to snap the Beyer headband pad around the headband - the length is a perfect match, so you won't be able to see any of your taped up Frankenstein headband.
The result should be incredibly durable, as the inner aluminum strip should hold against most of the flex, and the tape secures the outer strip to the top side of the headband with a small amount of flex, so it's not so uncomfortably rigid. The padding on the Beyer headband, against the slightly springy gap between the tape and the inner aluminum band (about 1/16" gap) is more than enough comfort for me, YMMV.
The materials cost should be less than $20, which is less than 1/4th of what it would cost you to order a new headband from Sennheiser, and the result should be far more sturdy than another frail headband that's likely to break on you again.