Just about any small HD recorder will do all you need, but I have a ZoomH4n and have used it to do something similar to this, so isd suggest a zoom.
Pros
Cost – cheap as chips
recording time – with a decent size sd/ sdhc card youll get way more capacity than youre going to use.
limiting – As its un attended, Id suggest setting up the internal limiter to take reasonable care of spikes etc.
size – very neat and unobtrusive / concealable
Cons
Will 2 record channels be enough?
Power – That record time wont be achievable on the internal AA batts. But you can power it mains or from a car 12v or a sep 12 v battery.
Linked audio gains. – Youll need to access and activate the software update to allow sep level controls of record tracks independently.
One last word of advice. NEVER 100% rely on any unattended recording. id give a regular eye to it, no matter how hard that is, OR get someone else to. You dont want to discover a problem at the end and have no audio on which you were reliant.
Id consider hooking up a radio link from the recorder to a receiver you have with cans so you can constantly hear what it is getting, and if there are power, audio feed, or major input over or underload issues.
Enjoy
Peter
Post Production Dubbing Mixer