Charger lead extensions
 

I use an HTRC B6 charger for mostly 3S lipos. Works fine for my needs. My only frustration is length of the charge leads, particularly the very short balance leads, which make it unwieldy to put the battery in a charge bag for safety. My plan is to build a charging box out of an old fireproof Sentry file box I have on hand.

Any reason why I can't extend both the main charge and balance leads (with appropriate gage wire) for the extensions? Probably need about 18 to 24 inch extensions for each.

No reason.
Buy two 12" JHS-XH 3S Balance Lead Extensions from Buddy RC and connect them together for 24".
They work great.

Balance lead extensions are available from many online sources including Amazon and Ebay. Many retailers also have them. Much cheaper than making them. No problem making the charge leads in longer lengths.