I think I’d agree with people saying that, with htis model, a box for her to ride in would be the best idea. On the other hand, if you really want her riding on his back, what about giving him a hunch back and letting her ride side saddle?
For details on side saddle design
http://users.tinyworld.co.uk/sidesaddlelady/1800%20side-saddle.html
http://www.damensattel.org/6Sattel/Teile/TeileDS-E.html
For images of woman riding side saddle
http://www.herefordequestriancentre.co.uk/side-saddle-gallery.html

EDIT: Sorry for bumping this from so far back, I should’ve checked the date of the last post before replying
