Try modified reverse cowgirl?
 

As a 300+ pounder, I've found that reverse cowgirl, with her straddling my right thigh (with her left leg under my left thigh) allows me to thrust best- I angle my hips to the right by placing my left foot flat on the bed, with my left knee in the air.

off topic here..... sorry..... 300 pounder?? you? really??