You can find the beasties ("Dungers" from Ramshackle Games, 5 for £ 3,5) here:
http://shop.ramshacklegames.co.uk/index.php?main_page=index&cPath=16
The Ork is from the GW Black Reach set; his legs fit the Dunger's side.
Heat the resin Dunger (in the over) to 80-90°C and cut a small corner from the top part of the beast's back (to fit the Ork's belly in), sculpt a saddle from Milliput or the likes, paint and glue the Ork in place. I plan a unit of "flame lancers"...
(http://i298.photobucket.com/albums/mm262/geudens_photos/mountedork03.jpg)
(http://i298.photobucket.com/albums/mm262/geudens_photos/mountedork02.jpg)
(http://i298.photobucket.com/albums/mm262/geudens_photos/mountedork01.jpg)
Rudi