DIY Li battery packs

Being a tight a**e, I was wondering if anyone has made up Li battery packs for a 6 volt minelab extreme. The battery I was looking at was a 7.4v Lipoly, but fully charged was 8.4v, which I presume would be too high for the electronics to handle. My batteries are getting due for replacement and I would like a smaller and lighter alternative to the lead ones, but I am far too tight to spend the $400 for the shop bought ones. Any help would be appreciated.
cheers
Peter

if you need some help with ur battery options would be willing to give you some advice if you still interested

Peter use a 7000 mah 11.1 vol you can detect all day
use a 12v to 6.8v or 7.4v reg
A LIPO charger you can pick up for $35
I used this on me gp extreme it worked well