I was looking for a smaller (shorter than an 18650) powerbank for very small and low-power applications.

I stumbled upon the T6845-C powerbank charger/booster/protector board (can be had for a few dollars) and it suited my needs sufficiently.

I used a small pouch cell that I had lying around, which was 38x25x6mm. You could use a larger cell if you increase the dimensions of the case, or can extend the battery-side of the box to allow a few stacked cells.

The material i used was clear 2mm acrylic (I haven't yet found a good cheap source for cast acrylic <2mm). Any glue-able 2mm material will work fine. Do note that if you want the LED visible, you'll need to use clear material, or make a light-pipe or something. One thing i sometimes do is use hot glue to fill a hole in MDF sheet, and the LED on the other side can glow through whilst being protected also :)

If I were to do this again, I'd remove the USB A connector from the PCB and flip it to the other side. This way the box will take up 5-6mm less with no consequence.

Note that for this PCB there is a charge current of ~1A, and if your cell is not OK with that, you can put an inline resistor (FROM INPUT SIDE) to limit charge current. My cell has a maximum charge current of ~200mA. So i chose a resistor as follows: Vflat = 3V, Vfull=4.2V, Imax=200mA. R = V/I = (USB-Vflat)/200mA = (5-3)/0.2 = 2/0.2 = 10R Imin = V/R = (USB-Vfull)/10R = (5-4.2)/10 = 0.8/10 = 80mA Pmax = IIV = 200mA200mA10R = 0.20.210 = 400mW (I used a 600mW resistor). Remember to place this resistor between the INPUT USB connector (micro connector) and the rest of the PCB. I cut the track from µUSB V+ pin and put the resistor there. If you place the resistor in series with the cell, it won't charge properly (as Vreg approaches Vfull (4.2-4.2 = 0, as opposed to ~5-4.2 = 0.8V), the current will drop too far. In addition, even if a schottky diode was used in the discharge direction there would still be too many losses.