Wow, I can't wait to see that working Jon. Be sure to post some pics of that thing. they can melt copper??? That is 1084 deg C , or nearly 2000 deg F . Very Impressive.
I've had a 12x12" lens for years -- Came with a warning "not a toy"!!
Serious heat and you can't stand to watch the beam at focus. Welding shields sure help.
The bigest cheap magnifier I found is a Carson bigeye 5" plastic decent lense.
I use is to solder under.
Burns alot better in Fla than here in Mo.
Sun power density magnified =50W^2cm , or .2W^2cm regular.
Med. supplys sells 8x10 freznel for $5
I'd like to make a big CPU aimed helistat field!
I built a "Solar furnace" from a fresnel lens back in the 70s - I used to melt lead with it, among other things.
Got it at Edmund Scientific - looks like they still sell the kit, but I'm sure you can find them cheaper. http://scientificsonline.com/product.asp_Q_pn_E_3052833