I simply use an external RC battery - a Turnigy 2650mAh 3S 1C sourced from Hobby King.
The battery discharge is rated at '1C' - this is fine for QRP use - and it doesn't need a specific storage voltage when not used, unlike many higher capacity cells. It is tiny and weighs in at just under 5.5 oz. I also bought a suitable charger from them, and with any lithium cell & I heed the manufacturers instructions on safe charging.
I added XT60 connectors - and most importantly an in-line fuse. I don't want the FT817 PCB to perform that task in the even of a fault :-)
I mainly use it for occasional operating /P when on holiday. The battery capacity is far greater than my kids patience, so I've never run flat ! But at this price, you could have a main and back-up battery in the to-go box.
The only problem I've had was explaining what it was to an airport security guard when leaving Cyprus - it went through the X-ray a few times !
The RC world has a lot of battery and connector options - all a lot more affordable than ham radio accessories, so it's worth taking a look.
5B/G4MAD boxed up - arrow shows boxed battery
EI/G4MAD. Foreground shows life saving fluids.
?
Paddy, G4MAD/P, EI/G4MAD, 5B/G4MAD
?
?
Paddy G4MAD
