The magnet does not need to be MB specific. GM trans have a 1 1/4" square magnet that is 1/16" thick. Others are similar. Dig around a trans shop junk pile or talk to your regular garage person. Even new it should be just a few $ at another brands parts dept.

Magnetic drain plugs don't have enough surface area but are better than nothing if you can't easily get inside the pan ( like a engine oil pan. )