I had some spare Undead (Tomb Kings) Chariots and a heap of Undead Archers, so I decided to combine the two. I used the horses from the chariots and carefully cut the archers from their bases, then super-glued them to the horses... and voila!
Tomb King Mounted Archers for Warmaster!
I'm probably going to mount them two to a base as these are skirmishers. Not so crazy idea after all!