[Budgetguy]

Is it possible to fit 40mm driver inside ath m50 with 45mm original driver?
If so what trick should I use?

 

[chlonengan]

Maybe you should add some sealant or gasket in between the clearance.
If you insist to use the original housing.

But I've actually been switching 40mm driver to 50mm. Rather than mod the original housing, I made new one with 3D printer

 

[Budgetguy]

I end up fit my 40mm driver into 45mm housing by cutting an infra-board into a donut shape and glued it to the exterior of my 40mm driver. The result is actually above my expectations and it fit well into the 45mm housing.