I initially thought ABS plastic would be a poor choice for construction material but when I thump on my Sony MDR-7506 or similar it's apparent that stuff is plastic although probably more dense. Other than the looks of the headphone I'm still fairly stunned how great the inexpensive Dayton drivers sound. Plasticine is used to cover the holes around the back of the speakers which means you could remove it, use more, use less, stuff with dampening material.
I guarantee you could tune these cans to your liking (somewhat). I did an A/B on my Crack with those DIY cans and Audio-Technica ATH-M50x and while the M50's won it wasn't by as much as you'd like to think. Especially when you start comparing price points.
I'm building a second pair now. This will be the experimental pair. I'm an old timer here who always took my vinyl sign cutter and personalized my gear with some weird play on words name.
Which is why I'm naming the 2nd pair "Beats Off"