That's the spirit!
I played around with the fit of the bolt carrier to the rails, made from 0.49" thick stock - it seems to fit OK. The only thickness issue is the ejector, because technically the bolt carrier sits above the rails. So the ejector slot that is machined in the bolt carrier fit on mine OK with the material. You may need to use a dremel stone to thin that part down slightly.
Here's where I got my tubing:
http://www.aircraftspruce.com/catalog/mepages/4130square.php
1x1 OD: 0.049" $4.400/per foot
I bought 2 feet of it, so for $12 I have enough material for about 6-8 rail sets, if my measurements are correct.
The next thinner size is .035". That's just under 18 gauge I think. Honestly, I think either size would work. The K-Var "factory" rails are thinner, about .030" Heat treatment of the ejector is going to compensate for thinner material.
If you are feeling adventurous, you could use normal sheet steel, then heat & bend it yourself...
http://www.aircraftspruce.com/catalog/mepages/4130sheet.php
Either way, just be sure you have a few spare reinforced cut-off disks for your dremel. It REALLY sucks breaking the last of your disks halfway through, then having to drive to Home Depot. Ask me how I know

Oh if you want to do it really really fast, you could use an angle grinder! I used my 5" grinder with a cut-off disk and zipped right through them!
What I did is to enlarge the hunterwarrior template in Paint Shop Pro until the printout measured correctly to the ruler in the program. Then I printed it out, used spray adhesive to stick it to the metal, then used it as a rough template for cutting. You'll need to clean the metal off with alcohol to get good adhesion. That helped me a great deal, vs. using a sharpie to trace out the lines.