With bike theft being on the rise around here, I find myself needing to carry many more locks and chains. These have a combined weight of 7kg and are a pain to keep in my backpack. I would also like 2/3 of my backpack back to carry other things.
I considered a tail pack, but me being the size that I am apparently takes up most of the seat. This means there is not enough room to mount one without it hanging off the rear of the seat, and not just slightly.

Are there any other known luggage solutions that work for a 78-81 DT125/175MX?

I have read rumors that the French got an actual rear rack, but I understand they are extremely rare to the point I haven't even found a picture of one.

The only other, and probably most realistic option is modifying a rack from another vehicle, but I am unsure which one would be the best fit. Without a picture of an OEM or even an aftermarket one I have no idea what the mounting points should look like.

If its not possible, then I guess it can be lived with. Just without any luggage capacity at all. Which is inconvenient...

Just an idea, Honda trail90’s had them. I think you could make one to fit.
Sneezles61

They were an accessory in the USA... many people bought them. I think all but one of the 78-on DT175's I've have came with the rack. Not that easy to find these days though.

There was a company in the US that made new ones a few years ago...  found the old link...  manracks.com  They appear out of business.

I haven't seen him on the forum for a long time but member Devin used to have racks for sale for several models...  I bought a couple from him.