A technicality you may be aware of, but 'optical' is the same SPDIF signal data transmitted via toslink rather than a wired coax connection.
You might look into using a coax/optical SPDIF converter between the AD2K and D-50. Not sure how well that works, but converters are small and inexpensive and it might fit better in the bag than the obvious route of going to a larger recorder with a coax SPDIF input (R44, PMD661,670,671, FR2, HD-P2, DR680, etc) I've never used a converter and have no idea how reliable they are. Although small this does introduce an extra set of connections and something else to power.
What about the Tascam DR110mk2, it's large handheld size and has a wired SPDIF input on a minijack. Not sure how well it works or if it works with the AD2K.
(some redundancy here, others posted while I was responding and got distracted)