You can buy metal key-ring clips with chains for about a £1.50 from cornershops and newsagents.
You can then clip the chains to fences to allow use as foothold and handholds. If the keyring clip is less than 7mm in diameter at it's thinnest - and most of them are - you can clip onto even the thin fences that they use at places like Faslane.

Bolting a piece of flat wood to the chain means the foothold stands proud of the fence and makes it is easier and quicker for several people to use in the dark. These are pocket-sized / foot-sized so are easily concealed and only four are required to help the least fit activists easily climb a 8m fence, and are also cheap enough to be disposable in a rush. If you are very careful attaching it to the fence then you don't even set off the high-tech vibration sensors that military fences often utlise, at least until you start climbing.

Make sure the wood you use is strong enough to take your bodyweight after drilling - ie test them once you've built them.