Tom - After about a week, the cloth (say it's green) will have turned into green powder in places and you will find it stuck all over the top and sides of the battery. If you prefer red, then a piece of red fabric will do that for you.
Harry - Today I bought a stiff plastic tray in a kitchen-wares store. One that is used to keep the knives separate from the spoons, forks, etc. I'm going to drill two holes in the bottom, turn it upside down, and hold it down in place over all the battery caps using threaded rods that are the same (but maybe longer as the usual ones that are used for securing the battery solidly in the battery box. I plan to hook the bottom ends the same way as for the battery bar rods. The venting from the cells will escape from either end of this design where the extensions beyond the ends of the battery turn down again.
I'll let you know how it works out.
And I'll be able to remove it in seconds before the TRA or VTR judges come around.