The "long bike lock" idea sounds good, cause you can lock your marker(s) together and even thread it through your kit bag.
But what about all your Loose gear. Barrells, spares etc?
You are going to have to lock everything up in your kit bag, then thread your marker to that, and lock it all to the table.(or nearest back player!)
Its a lot of hassle, but worth it in the end.
And this is taking for granted that the players area itself is "safe", and no non-players are allowed in.
This dosnt add any cost to your game fee, and the bike lock/padlocks are only a couple of quid.
My 2p worth.