This is the schematic and BOM (such as it is).

The scanner outputs RS-232 and XBee inputs RS-232, so there wasn't a need for a microcontroller (at least for now) -- the scanner sends ASCII directly into the XBee, which then transmits it through the air to any other XBee listening at that address. 

Notes:

 - The barcodes generated by sticker.dynamic.land are a bit small for this scanner.  It only scans them when held just so.

 -  The XBee runs at 3V and the scanner runs at 5V.  I used a 2xAA battery pack for 3V, with a step-up converter to turn it into 5V, and a level-shifter to step the scanner's data line back down to 3V for the XBee.

 - I configured the XBee "by hand" using the USB breakout board and the XCTU app.  I used channel C, PAN ID 74A0, MY address 1, destination address 2.

 - The only configuration change I made to the scanner was to set it to RS-232 mode.  (It defaults to PS2 mode.)  This is done by scanning special barcodes in the quick guide.

 - The battery case has an on-off switch.  I haven't measured how much idle current the devices draw.

 - If we were to make more of these, we wouldn't use the USB breakout board in the design.  I used it because the XBee device itself has a narrow pin spacing which doesn't fit into a perf board, and the USB breakout board was the only breakout board I had.