Welcome to the forum. Plenty of take down air rifles available if PCPs are your thing. The air arms TDR has quite a following.
Otherwise bullpups with the mods removed can be very compact. Ugly dogs, all of them though.
If you're after some character, plenty of take downs from the collectables stable. Mostly inspired by day of the Jackal so proper interesting.
If you're into springers, most don't take a lot of effort to remove the action from the stock.
Failing that, modified guitar / trombone case? Avoiding of course any action that could lead to concealed weapon territory.
Dave