Hatha yoga pradipika contains posture details of Siddhasana as follows
Press firmly the heel of the left foot against the perineum, and the right heel above the male organ. With the chin pressing on the chest, one should sit calmly, having restrained the senses, and gaze steadily the space between the eyebrows. This is called the Siddha Âsana, the opener of the door of salvation.
It explicitly makes one point: one has to use the left heel.
Are there any other statements that accept the right heel against the perineum?