@independent, what I did was lifted both of the regulators so the leads were exposed.
I then snipped the leads of the add-on cell at the regulator circuit board.
I slipped shrink tube over the flat leads coming form the add-on cell.
Finally I bent and soldered the leads of the add on cell to the regulator circuit board.
Sometimes I trim plastic to make more room, sometimes I need to remove the bottom plastic strip so the battery does not cause the GSM to become disabled.
This still generally needs some tweaking to get the phone to recognize the battery, just adjusting the angle of the plastic housing for the battery regulator circuit, insert, and try to start.

It could be that my phone is getting old but I do not remember having any of this getting-it-to-work hassle with Scuds and I always got at least 3000mA/h for several years.