Instead of spending $28.53 on buying the official repair kit you could just print your own and perhaps customized sliders. Guaranteed more robust than the ones you are already attempting to replace.
Should be compatible with the following headphones:
DT 790 / DT 797 / DT 770 / DT 770 PRO / DT 770 PRO HT / DT 880 / DT 880 PRO / DT 880 PRO HT / DT 990 / DT 990 PRO / HS 200 Dyn / HS 200 Rotor / HS 200 Trend / HS 400
This make is only a replacement for the cover, not the actual slider mechanism.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/eMUzTFdYo8H
Simply use the original "slider thingy" that fits inside the 3d print.
First design uses the original screws.
Updated design uses M3x12 flat head screws.