A canvas cover secured on one side, and with velcro on the other would do. And you can have the canvas stamped with some sort of message that would deter theft. for example, the words, caution, radioactive materials, keep cover in place.. That would deter most people.
Why am I not surprised at this suggestion, Tom? Perhaps a glowing light seeping through the canvas would remove any doubt from such a scallywag? HA! (Thieves are smarter than that today...)
While living in Dallas during the late 70's, I did hear of a poor fellow who left his photographer boss's equipment in his trunk overnight after a late shoot only to find it all gone the next morning - it must have been an inside job.
I will be throwing a heavy blanket or other nondescript cover over this box to hold down the contents and bring no attention to it. Spring will be here before we know it, too, so I'm ready for the first photo opp in the Smokies!!
Common Man Woodworking