[Suggestions for stock extension for Specna Arms]

Hi all. First time poster and looking for a bit of advice please!

I've got a Specna Arms Sa39-Edge. It's really tricky to fit the battery into the short stock. I wondered if anybody could recommend a generic extension to make it easier and to house larger batteries? There is Specna Arms model but it looks to be out of stock everywhere. 

Thanks very much in advance!