I think that it may be best if you position the transmission case so that the filler and its hole are angled slightly downward (so chips are less likely to fall into trans) and try drilling out the cap.
I assume its made of plastic? If so, you may need to drill several , "intersecting, or overlapping" holes in the cap to provide you with a means to insert a flat screwdriver blade to help you pry out the stuck halves of the cap.
Has the cooling fan been damaged or broken on the input pulley?
Was there a lot of debris built up on the top of the case?
There had to be an underlying cause for temps to reach the melting point of plastic.

Fan blade was not broken and debris was at a minimum. When I took trannyout the filler plug was actually split on top and was letting fluid to leak out a little so I was glad I decided to take the tranny out. I've drain it and did like you said as far as keeping in tilted so debris could fall back in the tranny. But can't get enough of the old filler cap off so I can even get magnetic out. Was going to wait until new obw arrives tomorrow to see what it looks like so I'll know what I've got to dig and scrap out

just MHO... but tranny had to see some hi temps for that plastic to melt and deform - makes one wonder about other seals, etc. ...