You could still use a bpay band ( take out the chip from inside to make it smaller) and top it up with Monzo, but it would be nice to have it directly linked.
https://www.bpay.co.uk the fob and wristband open up and you can take out the biggest simcard sized chip and put it somewhere else if you want. (Maybe other products open I’m not sure)
Also about buying from eBay, these chips expire so make sure you can see the date on the pic or you might get one with a few months left.