Looking for ideas to make wire connectors compatible with Lego Functions
The only option comes to my mind is to buy LEGO Power Functions Extension Wire # 8886, cut the wire, and connect it to my custom battery pack. Would that work?


Yes it would work, and people have done it.

Here's a YouTube video someone has made on their own custom Power Functions batteries.




  Is there a better idea?


I don't think it would really be possible to produce a Power Functions compatible connector for less that the $3 LEGO charges for their cable at a small scale, so that's probably your cheapest option.