PacSafe Stuffsafe & Lidsafe
I recently bought both of these and would recommend them to deter casual theft. If you have memories of walking around historical sites in the heat wearing your hot jacket, or carrying your helmet or one boot then these could be the answer.
will keep your helmet secure without taking up space in a top box. It's well made with a steel mesh inside the material so there are no rough edges to damage the helmet. The outer material is also waterproof. If you have a second one for pillion riders you can also keep a spare helmet strapped on the seat without worrying too much about losing it.
is a bit fiddly to begin with and, unlike the LidSafe, the steel mesh is independent of the material stuff sack. Hence, I discovered that it's easier to fill the sack separately and pull the mesh over it when filled out. It will easily hold a pair of MX boots, a bulky jacket and a helmet or two.
Even better for me, I use 2 SealLine Dry Bags
on my bike. (I think they are the Baja 20.) One is for my tent and the other is for sleeping bags et cetera. Both of these bags, when full, fit side by side into the Stuffsafe which gives the option of securing all my sleeping equipment if necessary.
The Stuffsafe also comes with a roll-up carrying pouch and belt. Since the steel is very flexible it can be worn around the waist as a means of carrying it.
To me, the weakest point of these products looks to be the padlock but that might be because I am used to seeing poor quality ones. I'd like to think that Pacsafe maintain their reputation with high quality padlocks which equal the security of the steel. If anyone has tested their padlocks let me know.
As mentioned, these might not stop a professional thief who has targeted your bike but they are great for when you just pull up in a new location and have to leave the bike unattended.
For best prices, avoid specialist shops and look on the Internet, e.g. eBay.