Laser tools make drain plug adaptors along with various other manufacturers. These are a good investment as they are designed specifiacally for the job and are made of good quality materials and fit most major brands. Normally they fit a socket wrench too
We've all got old allen keys in our garages from dubious origins that are poorly made from crappy steel and normally a rubbish fit. I've got a box full of these and most haven't even got a size stamped on them!
Using something like this is the best way to ruin drain/fill plugs!
Plug adaptors aren't a glamourous tool, but for a tenner they save a lot of bother in the long run.
When you fill the box with oil, i think you fill until it just starts to trickle out of the fill plug.
I'm assuming your clutch is dis-engaging alright???