We finally got our hands on some nano-USB memory sticks, and have adjusted the radio placement in the MR3020 routers we are using, so that the lid can go back on without duct-tape, and the whole unit looks much sleeker.
Also, we have some smaller and surprisingly complementary styled rechargeable battery units for them, that even have a percentage charge remaining indication. All for $17.50 on ebay!
The result is as shown in the following photos.
The battery packs even have a 2-digit LED display to tell you how much battery capacity you have left -- very handy for in the field.
The ability to directly wire the RFD900 into these units simplifies the wiring amazingly, and really is one of the big enablers for being able to assemble the prototypes at an affordable cost: