How Can Difficulty In Intercourse Due To Excessive Weight Be Handled?

I am a rather large man, and a touch over 6 and about 300 lbs my wife is barely 5 tall with very short legs. we both like her on top from time to time, and with her legs so short she can not reach the bed or floor with her knees, thus she can t mount me and if we do connect she has know way of support or ant leverage to move any way she wants or needs. We have tried some different home remedies for this from pillows to a few other things to help her reach and they either turn out to be uncomfortable or slide all over making it nearly impossible for her to support her self or to move as she needs. Is the any type of device that would help us with this problem that would make it possible for her to have enough support and be able to have sex in this basic position?