The robust wire I  used for the external battery connection required a bit of diagonal cutter work to enlarge the hole in the top plate, but eventually everything fit together and the GPS interface box latched neatly onto the radio:

HT-GPS Case - cabled top view

The skein of cables:

• Antenna: reverse SMA to UHF adapter = RG58 coax
• GPS: TTL serial data from Byonics GPS2 receiver = DB-9 (OK, DE-9)
• Helmet: mic + earbud = repurposed USB cable
• PTT: 3.5 mm jack = repurposed audio cable
• External power: 18 gauge zip cord + Powerpoles = repurposed speaker cable

All in all, it looks pretty good:

HT-GPS Case - cabled and powered

After a few rides to verify that this whole affair works, I must print up another case with slightly modified dimensions, add a plastic window over those cheerful LEDs on the TinyTrak3+ board, and mush an epoxy putty blob over the earphone and mic connections on that bright yellow plug plate. I’ve given up on the idea of having a cover for the top part of the battery compartment; there just isn’t enough space for such a thing and it’d be an impossibly delicate shell.

The radio seems happy enough being fed 9 V from a bench supply (to match the upconverted lithium packs I’ve been using on the bikes), rather than 7.4 V from its standard lithium pack. A freshly charged battery comes pretty close to 9 V, so they can’t be too fussy. It idles at about 100 mA, with periodic blips to 140 mA when (I think) the TinyTrak3+ tickles the GPS receiver, regardless of the supply voltage. Goosing it with 13.8 V surely wouldn’t have a happy outcome…

Lashed up like that on the bench, with the GPS receiver hanging out the basement window and the coax hitched to a bicone scanner antenna sitting inside the window, it generated APRS spots and the audio sounds OK, so the innards look good, too.

2012-04-11 20:09:08 EDT: KE4ZNU-9>APT311,WA2YSM-15,WIDE1*,WIDE2-1,qAR,K2MHV-6:>Ed - Bike PL100 UV3D

One downside: the TinyTrak3+ blurts its initial ID message instantly after being powered on, but the radio takes a few seconds to haul itself to its feet. As a result, the ID message never reaches the antenna. So it goes…