You can also search here at BPL. Try pyramid, teepee and tipi as search terms. You could talk to Tarbubble (Colleen) about her experience. (she runs the Backpacking with Kids Yahoo group)
Also check this thread, and pay particular attention to what David Olsen and Ron Bell have to say, since they both make and sell them.
It's perfectly feasible to copy a Megalite but, whilst it's reasonable and pretty cheap, it's not the greatest design ever. As David says, catenary cut will make the design MUCH better.
You may want to post a WTB on Gear Swap, since a good used one will likely only run you $150, and you can use it to learn what you like, then sell it again for about the same amount. There's about 10-15 yards of silnylon in one, so it's worth looking SH.
Also check Roger Caffin's info on sewing with silnylon.
And don't forget to post pics!