Walter got some ultra cheap USB power supplies from AliExpress. The price was slightly too good to be true, so brought one along to RevSpace.
The biggest problem appears to be that a 300+ V DC line runs perilously close to the USB connector (near C4). Probably not something to use in moist environments, such as the SparkShack. Or maybe if you want to see sparks, one to use in such an environment.
Equally lovely are the lines near R1 and R6, in terms of their proximity. This whole thing could easily be a magnitude safer if the rectifier (D6) had been turned ninety degrees counter-clockwise.