Since kernel 6.13 (~01/2025), there is a new makefile argument: MO=<build-dir>
make -C <kernel-dir> M=<module-src-dir> MO=<module-build-dir>
(see https://www.kernel.org/doc/html/v6.13/kbuild/modules.html#options )
The (final) patchset, for reference: https://lkml.org/lkml/2024/11/10/32
Enjoy.