I think the box will be fairly small anyway, so if it'll fit into the 200sx, it should go into the G20 no problem, dont worry about it. When ive designed it, i'll give you an overall size, so you can judge for yourself.
There are however, serveral tools, you'll need. This is what i have and would use.
Wood saw
Cordless drill/screw driver
Sander, flat bed or orbital
Jigsaw
and if possible, a power plainer.
The power plainer helps you get the correct angles when joining the sloping side to the box.
and dont forget a good pencil, ruler and tape measure.
