Does anyone know where to get a waterproof cover for the USB port/connector on the USB WERK?
I doubt that there will be one.
The circular plugs are waterproof, but USB ports aren't.
You could look for a USB-A socket on a long lead to a round plug, or a round plug to round socket extension lead, that would allow putting the socket in a handlebar bag.
An extension is part of the e-Werk cable set (£21). I've not seen it separately, but I've not looked.
Alternatively, get a length of inner tube that's fat enough to accommodate the socket, put the lead through it so the USB socket is recessed inside by about an inch from the end, then zip tie it to your head tube, folded over in the middle so both ends are downwards.
Should be pretty rainproof so long as you use mudguards. An advantage of this is that any water that does make it into the socket will dry off fairly quickly. The same can't be said for an attempted waterproof cover that's leaked.
If you don't have any dead tubes that are fat enough, Poundland sells inner tubes that were fat enough for my Igaro D2.