For anyone who is wondering about the lights:

Amazon DE: https://www.amazon.de/gp/product/B07NL1J9V4/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Amazon com: https://www.amazon.com/SOLMORE-Waterproof-Backlight-Battery-powered-Decoration/dp/B073BC8WYW/ref=sr_1_8?keywords=Battery-operated+LED+Strip+Lights%2C+24+Buttons+Remote+Controlled%2C+DIY+Indoor+and+Outdoor+Decoration&qid=1566386367&s=gateway&sr=8-8

Amazon UK: https://www.amazon.co.uk/Battery-Powered-Controlled-Decoration-Waterproof/dp/B07MTY1F2T/ref=sr_1_19?keywords=Battery-operated+LED+Strip&qid=1566386479&s=gateway&sr=8-19

and for the corners i used extensions/quick connectors i already had. But something like this should do it:

https://www.amazon.de/LitaElek-Schnellverbinder-Eckverbinder-Verbindungskabel-Anschlusskabel-Rgb-5050-Nicht-Wasserdicht-Verbinder-10pcs/dp/B01EJ32EXO/ref=sr_1_9?crid=1DV7KKWKB5UPV&keywords=led+schnellverbinder&qid=1566386621&s=lighting&sprefix=led+quick+%2Clighting%2C159&sr=1-9

There are a lot of connectors like this you just need to search for the ones you want.

For the bottom of the battery pack i used strong double sided tape and to clean the corner wires i used black electrical tape.