The battery that'll fit snugly into my project box is 5.28 x 2.64 x 2.38 Inches. For the charger I can afford something the size of an UNO, give or take a bit. The box will have two tiny cpu fans to bring air in and hot air out, so the unit should not contribute to overheting the whole business.
But I need to come up with a number of these units, so ideally it would be an off the shelf product. When using a 20W panel it puts out around 1.2A at 18V.