I thought it would be sort of entertaining to have an eidolon that looked like a crab with a straw thatched hut on its back. The character would sleep in the hut and fight in from the hut when the creature got big enough.
The problem is, I can't seem to find a way to make this work. The easiest method I can find is to use the synthesist summoner and refluff the fused eidolon ability as the character walking into the house, but then nobody else can go in the house. There's also the Howdah, but they require a huge creature, so it would be awhile before the creature was large enough to carry a howdah, and I really wanted it to be part of the creature. At that point I'd be better off going with a witch's hut grand hex or an animated object.
Is there a good way to do this?
You could use an exotic saddle I guess when it's smaller than huge size, or use the rules for vehicles (though those are a pain IMO.) I don't think there's a built-into-the-eidolon option anywhere.