Yours looks like my conversion only a little rougher around the edges. Polycarbonate is easy to cut with a regular hacksaw. You can file and shape it just like dense wood. Only it tends to load up so you need a wire brush to clean the plastic out of the file. If you take a little time the edge will come out nice. I find that I didn't need the wall in the rear of my battery box. Just to save you some weight.
You asked for weights of trucks, mine started out at 12lbs as seen here.
I would also reccomend
Husky Hangalls to for the battery.
I also now run my box mount under the chassis tabs. That way the force of and impact is spread over the area of the tab and not on the screws. All you need is some thin foam. I used a high density packing foam. It was just about 3mm so it is about flush with the tab.
Here is my truck as it was. Right now it's being rebuild from the ground up to better support it's new bashing duties.
http://www.rc-monster.com/forum/showthread.php?t=6847