I believe you would only need the thick back cover if you plan to upgrade the battery to a 2000mAh version.
I do plan on it
If your Gen. 5 is a thin back and you plan to keep using the stock battery, I see no reason why the iFlash-Dual wouldn't fit, however I've not actually tried it and Tarkan would best be able to answer the question of whether the Dual will fit in a thin backed iPod.
Yes it's the 30gig 5th gen
Here is a
link to that 2000mAh battery, from the most trusted seller. Other sellers have proven to be more problematic to some of the members on this forum, so I'd stick with this listing if I were you.
There are NO US based sellers of this battery??? I hate waiting for the slow boat from China...
You might also want to pick-up the thick version headphone jack/hold switch assembly too. I have successfully done without it and just reused the existing thin version in one or more of my builds, by placing a small mount of foam as a spacer, but other members have cautioned not to do this.
This jack is from the 80gig 5.5gen????