I had to replace the Servo back up batteries a while back and found that the recommended batteries that fit in the little yellow holder worked out really expensive.
https://bestbattery.biz/index.php?optio ... oCmIjw_wcB
So I got a pair of these...
https://www.hardwarexpress.co.uk/csb-gp6 ... oCpYLw_wcB
I soldered some spade connectors to the connector that i cut off from the old battery and cable tied them in.
Worked out at £22 delivered, roughly half the cost and double the Ah of the original batteries.