Presumably the size of 2 12v 12 ah sla's. Most likely you can fit 5 ah 5s lipo bricks into the space. You'd have room for 6 of em, giving you 15s lipo, 10 ah.
It will be tight on the length, but not so tall giving room for the wiring harness.
Cost won't be so cheap but it's still likely to be the cheapest, A good charger is at least 100 with it's power supply, then the lipos are at least 50 apiece, then some shipping, and other stuff for the wiring harness. $500 or so. But you could do a starter pack of 5 ah cheaper.
Woth it for sure.
Another fine option would be to build a battery, or have one built. Cellman is the guy to get the A123's from. http://www.emissions-free.com/index.html
Perhaps a bit more pricy, but in the end a bit more plug in, forget about it, unplug and ride.
There are many options for ready made bike batteries in 48v. Notably Ebikekit, and Ebikes-ca. http://www.e-bikekit.com/shop/index.php?p=product&id=121&parent=2 Those are pretty similar in shape, and might fit the battery tray of the lashout. A battery from pingbattery would work too, but doubtfull that it would fit in the box.
Lastly, there are various lithium power tool batteries that could be connected up to make a 48v pack. And there are guys in the for sale section here, that sell those cells after they have been removed from the packs, and the bad cells weeded out.
