Box on the trailer no good now - the whole world knows where you hid the key!
Ref. the bikes I have also put them against the console. Removed the pedals - makes them a lot eeasier to stack.
Polys Idea with the mechanical key is potentially good.
Only thing to bear in mind is read your car's handbook. I used my key mechnically when the batteries died in the remote & sod's law nobody had the right ones in stock. The alarm may behave a little differently when locked / unlocked by key. Also a lot of the cars these days use RF coding so you may find it locks but doesn't set the alarm etc.
Worth a test before you find you can't get back in whilst camped in the @rse end of nowhere....