about.

I wanted a permanently installed, battery powered rear light on my bicycle. I wasn't happy with an attachment on my seat post, as my thighs would touch the edges of the light.
To get good visibility and also have it out of the way, attaching it directly underneath the edge of the saddle seems reasonable, as long as the saddle allows this.

Basically, a holder with a GoPro-compatible mount gets attached to the rods on the saddle, and a little GoPro-compatible offset arm attaches the (obviously also GoPro-compatible) light-holder to it. It can be adjusted and tilted, multiple offset arms (or different, longer ones) can be combined as well.
You can also attach any other GoPro-compatible accessories, or even a GoPro itself.

I hate to de- and re-attach bike lights for every ride with a passion, but the light is cheap enough so it won't ruin me if it's stolen. I also recommend having small backup-lights with you in case it indeed gets stolen, or the batteries are drained.

BOM. Things you need.

• 2x 25mm M5 bolt countersunk head
• 2x 20mm M5 bolt countersunk head
• 6x M5 nut
• 2x M5 washer
• 1x bike light with attached M5 bolts

I strongly suggest to use screws that are a few millimeters longer and use locking nuts to make it more secure.
That is the base-config; add a 20mm bold and a nut per additional offset arm. I got the light for cheap from Action.
The bolts on the may have a center-to-center distance 66mm to 88mm (80mm is very common). The rod-distance on the saddle needs to be 44mm center-to-center (which seems to be the standard)

printing advice.

I strongly suggest printing in PETG or ABS; better not print in PLA. If this breaks during a right, it can potentially be dangerous to people around you.
No supports needed.