Additional accessory mount, focused on flashlight mounting.

180 degree clamp attached to head tube (50mm diameter) using 3M VHB tape.
Light attached to extender using standard 30mm clamp mounted used in bike accessory.

There's also 25mm hole to save filament which could be also used as holder for single 18650 power bank. This allows to increase built-in 700mAh battery with another 3400mAh (approx.)

Head tube = 50mm
Light mount = 30mm
Inner hole = 25mm

Used flashlight:
Rockbros HL1704BC1101
https://tinyurl.com/y7od8t8m

Power bank:
Common chinese case for 18650
https://tinyurl.com/ycprrmwu
Power bank diameter is 20mm, hole is 25mm, it's necessary to insert 5mm of rubber material for example self-curing tape which blocks vibrations and holds power bank in place.

Links are random, i don't recommend those seller as i don't have experience with them.