It's alive! (HP 25 w/Panamatik ACT)

[Luro1964]

Thanks to Bernhard and the info on this forum, I'll soon be attempting to revive my dad's HP-25. The chip has arrived, will read the instructions carefully!
My question to you all is this: some corrosion from one of the batteries has eaten away at the black cardboard (?) separator at the terminals. I am not sure actually what its function is and what it is made of. I would need to make a replacement. What can I use?
Thanks in advance for any advice!