Hmmmm - on a carbine, I would check the total costings first......
A rail/ris that may or may not fit on easily in place of your handguard
peq box will deffo hold smaller battery as well
so add up the ris + peq + peq battery - could work out not a cheap & easy/perfect solution
try and work out what "might" fit in there in that handguard as it stands
don't matter how you do it but YOU have to check if something might fit in there or not
I don't have a G&G carbine - ICS M4A1 but it could be a tad different to your gun
try to guess and make up something that might be a similar size to a smaller thin block battery
http://www.hobbyking.co.uk/hobbyking/store/uh_viewItem.asp?idProduct=31975
I AM NOT SAYING THIS WILL FIT !!!!!!!!
YOU HAVE TO TRY TO CHECK - use any sort of box/carboard/even bits of wood/kids wooden blocks etc....
make up something a tiny bit bigger than battery dimensions of any battery you might be looking at
and see if she fits in your current handguard/battery compartment
You could buy say a twin stick nunchuck lipo but a single block battery
or perhaps short stick battery is nearly always cheaper as they are more common in RC stuff
The one shown is not too long and quite thin - might be a tad wide though but in any case YOU need to check yourself
if you do a little bit of research you may find a cheap decent block battery that just fits for a lot cheaper than you thought
There comes a point in everything - where do you start & where do you stop.....
You could do this, that & the other, rewire blah blah blah
or you upgrade to one with rails already blah blah blah.....
You may just be able to fit in a lipo block in there if you do your homework, very cheaply and not have to go nutz
then save the money and perhaps upgrade later at some point if you still need to
up to you but just giving you a few ideas/thoughts
please double/triple check any battery you buy will fit in there - add a couple of mm to each dimension coz they can vary a smidge
you might get this one in there but is a bit thicker:
http://www.hobbyking.co.uk/hobbyking/store/uh_viewItem.asp?idProduct=51354
so please check as it is YOUR gun/battery not mine