As an outside possibility, how about a readymade unit and plugging in with micro USB?
This might work well in a clear lid enclosure with a sachet of silica gel in the enclosure for good measure.
Do a search for “5000mAh USB Waterproof Solar Power Bank Battery” on ebay.
On a Spark core consuming 50mA this would give about 70 hours runtime (being 3.7V lithium battery at 5000mAh). Clearly longer on a Photon where cited power consumption is lower than 50mA.
Building your own would enable more control over deisgn parameters, and may work better in winter conditions with insufficient daily charging light for the unit to cover 24hr consumption by your circuits. Although in sunnier climates than the UK, that may not be a problem.