Just to add to this old topic since I see if be the first to comment about this particular option...
As a vagina-haver (meaning unlike with a penis where the pee goes a bit into the front of the diaper, it goes much more directly down between the legs for us), I prefer laying down on my tummy or on all fours, bc I like for the warmth to gather where I can rub my clit through the squishy warmth of it after, and when laying on your back it gathers pretty much entirely around you ass which just isn't as satisfying. So basically I like to use gravity to my advantage lol.