This mod wasn't actually done to a motorized bike, but it should apply the same. Many of us would like to do anything to make our MBs look more like motorcycles, and I think adding cargo capability definitely helps. This is an especially good mod for those who's bikes are painted army-green, like my own MB.
Ammo boxes are also relatively cheap and there are many of them for sale in pairs or more on eBay.
A warning to those looking to mod their motorized bicycles:
Most, if not all, of the hacks and mods featured on this site were made by individual hobbyists. If you are new to working with 2-stroke bicycle engines, please by aware that there's a possibility that a mod featured on this site could seriously damage your engine. Please don't try any mod from this or any other site unless you fully understand what you are doing. The owner of this blog will not be held responsible for both material and bodily damage caused by performing a modification featured on this blog. Also remember that opening up your engine may void your warranty!