I had the same problem on my 300e. I took it to my mechanic
and explained my frustartion. He used a special rachet key
that was designed specifically for stripped heads. Luckily,
he didn't charge me anything. I guess he felt sorry for me
and could easily read the frustration on my face.
I would suggest taking your car to a " Lube Shop ". Since they do
oil and filter changes on a regular basis, I would imagine they
run into this problem all the time. Ask them to remove the
drain plug for you and then just reinstall the new one yourself.
Hope all goes well for you,