I paid around $470 including UPS shipping (bidding rather than buy now so saved a bit) for the 36V 20Ah battery.
I got a small top box off eBay to fit it into, fits straight onto the standard rack. I had to add another refllector- the original is hidden by the strangely shaped box- it's not flat on the bottom so hangs over the back a little. It leaves some space between the seat and the box for a bit of luggage though. Since buying off eBay I've seen the same one for more money and with other names including Raleigh.
It's like
this on eBay
The standard battery cable has been re-routed and fed through a hole drilled into the base of the top box (I drilled it large enough to feed the connector through rather than remove the connector from the cable and feeding the cable through a smaller hole. Maplin sell blanking and cable plugs to seal the hole to stop water ingress, I ended up using a blanking one but need to redo one, the first attempt works but could be improved.) Maybe removing the plug would have been better but this was easier.
I got XLR connectors (Maplin) which are the battery connection ones on the standard battery and wiring to fit onto the LiFePO4 when it arrives leaving the X-Byke bits as standard so I can use the standard battery if needed.
I'll post some pics when the battery arrives and it's all fitted.