[talliedthoughts]

Ask for a second key and leave it in the slot?

[kingofpandora]

Those slots don't require a specific key. Put your library card in there and it'll work fine.

[Scoundreller]

They get suspicious when I pay for X occupancy but request X+Y keys.

[ghaff]

Hotels routinely give me two cards when I check in--and certainly will if I ask. I usually stick one in my wallet and one in my bag or whatever so I'm less likely to accidentally leave the card in the room.

[sawmurai]

Usually you can stick anything that has the correct form into that slot. Even thin cardboard :)