Here's a simple adapter using my favorite DIY material for mounting stuff on a bike - PVC irrigation pipe. Anybody who lives in the desert where irrigation is common has short lengths of this stuff lying around to be scavenged but you can get a short piece at any Home Depot. You can paint it black to match the seatpost. Aero seatpost light mount Aero seatpost light mount - Slowtwitch.com With it you can mount one of the ultra cheap but very bright COB LED taillights with a built in lithium polymer battery. 100 lumens for under $5 postpaid USB Rechargeable LED Bicycle Bike Cycling Rear Tail Light 6-Modes Lamp US | eBay or this Raypal 168 lumen light identical to the Blitzu Cyborg but cheaper at $12.25 on Amazon https://www.amazon.com/Rechargeable-...light+RPL-2266

I own both of them and I can guarantee you they are MUCH brighter than anything that runs on button cells.