I'm not at home at the moment so i can't post a picture of the drawing i used to work out the new frame work. However if you go back through this thread a few pages you'll see a few pics of my frame drawn on a white cardboard sheet. I've used the same method for 4 frames now - Rodgar's BigHit, Hyena's Diamomdback and my two and its worked a treat. Best thing is its about as simple as you can get!
Firstly, make sure you know what batteries you want to use and what their dimensions are. Add a little more for wiring and slight puffing.
Then carefully measure the frame from the BB to the head tube to see how the batteries will fit, allowing for the frame to bend in towards the head tube. You want the batteries to finish just before it starts bending in at the front (the bend about 60 - 80mm from the head tube).
Next, get some cheap sheets of white cardboard from a news agency or department store. Take some photos of the bike from the side for reference. Strip the bike down to the bare frame and carefully trace out the frame shape onto the cardboard. Use a square if needed to get an accurate trace. If your going to use the same forks, this makes it alot easier as you can just extend the frame at the same head angle on the drawing.
Add in your new section of frame on the drawing, rubbing out the old once you've finished. This will give you the dimensions you need to cut the original frame at while also giving you the new dimensions and angles you'll need to cut and weld the new insert.
I've had a few rums tonight so my explanation may be a little jumbled but hopefully it makes sense lol
Getting it all down on some paper or cardboard is the key to it all coming together as planned. It may not be as flash as a CAD drawing but its simple to do adjustments on paper and get measurements off for when it comes time to make the new insert
Finally, check your finished drawing before cutting anything! Then double check it again and again. Make sure you've allowed for material thickness where needed and how your going to place the panels in the insert for welding (edge to edge or edge to flat etc).
Oh and 1/8" should be fine. I've used 3mm on my last 3 frames and its been ok. If you want a little more strength, you could add some 4, 5 or 6mm (1/4" plate) to the bottom piece. Just bare in mind the weight penalty that comes with it.