About the APC UPB10; I haven't used it with the 305 yet as I just haven't had the need. I know I will, especially on rides which span multiple days with camping in between where there isn't access to plugs to charge the 305 up. The external battery works well for any usb charged device (my cell phone can be charged this way too) so it's great for multiple uses. My thought was exactly like yours about using it with my bento box. It should be a Med. - Large bento box in order for it to fit if you don't want it sticking out the top.
The APC UPB10 is about the same size as my Palm Treo 700 which I carry easily in my Bento Box. So, it looks like a good option.
Thanks for the feedback. I will be buying the Garmine 305 Edge shortly and will let you know how it is working for me.