3M Dual Lock Battery and Multi Purpose Mounting Tape (25mm x 1M)

3M Dual Lock Fastener SJ3551 With Self Adhesive Backing. 25mm wide.

Now supplied in 1M length (was 500mm) More than enough to mount 2 batteries

Stronger Than Hook And Loop Fastener
This is designed to fix to itself so there are not 2 different sides.
If you needed to attach something 100mm long you would need 200mm of tape.