I made use of the 10% off eBay code on Friday and ordered https://www.ebay.co.uk/itm/36V-13Ah-E-bike-Pedelec-Rear-Li-ion-Battery-Carrier-Bracket-w-Lock-Charger/113217309391?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
However, the rack is for V-brake bikes and I've browsed eBay...