VELCRO Brand VEL-EC60214 White Stick On Tape 20 mm x 2.5 m Roll https://smile.amazon.co.uk/dp/B0013DO888/ref=cm_sw_r_apan_glt_fabc_ABJ4C5MDAWF6K5YNK8P2?_encoding=UTF8&psc=1
I got this stuff from B and M - it's great. I've stuck the battery boxes to the top of the window frame with the lights hanging.