OK, guys, here's what I did...
a peice of flatbar, stainless steel, 1.245"wide by 5/8" thick, 13 inches long. cold rolled steel would be fine, but this is what I had at hand.
same 13" length of 1.250" square tube, stainless. again, a length of good, heavy cold-rolled, hot-rolled, carbon steel square tube, or heavy 1-1/4" channel iron will suffice for this piece.
I laid a TAPCO flat onto the flatbar, eyeballed center, and marked all of the openings in the flat onto the flatbar with a Sharpie marker. I then found center on the flatbar, and drilled 6 holes down the length, so when I stick a bolt through, the bolt will go through the flatbar, and through the existing openings in the TAPCO flat.
I then laid the flatbar on top of the square tube, evened up the ends, and transferred these drilled holes to the square tube. The square tube got drilled with a Q letter drill, and tapped to 3/8" fine thread. The holes in the flatbar were drilled out to 25/64".
The flatbar received a couple of notches on the side to accept the dimples of the flat when it is rolled up. the dimples are a bit oversized, approximately 1/4"deep, and 3/4" long. I did this on a mill, laying the flatbar down in the vice flat, and drilling a 3/8" endmill down so that a portion of the end mill was hanging over the edge, stopping about 1/8" from going through the back, then plowing out a slot about 3/4" long. I did this on both sides.
I flipped it over, and milled out a portion to accept the magwell lip. this step and step 6 could have just as easily been done with a dermel or grinder, but why not use the mill when you have one nearby?
I took the TAPCO flat, and laid it onto the square tube, aligning the holes, and then laid the flatbar onto the TAPCO flat, aligning the recess for the magwell lip and the drilled holes, put the bolts in, and finger tightened.
using my calipers, I measured, tapped, and re-measured, centring the flat exactly on the flatbar, then took up some torque on the bolts, and re-measured. I cranked the bolts down really tight, and measured one last time. I now had my sandwich.
I stuck the sandwich in the vice, cranked it down super tight, and using a 2x4 scrap and 32 ounce ball-pien, began tapping the edge of the flat down, going back and forth, checking to make sure it was going evenly. I think I like bending the edge this way for a couple of reasons...
the finished result is a very sharp radius, folded over the edge of the flatbar, not stretched into shape. there is very little, if any stretching of the metal. you can also see the progress at all times, and can give your attention to one small section at a time if you need to. you are not blinded by pushing it down into a die.. when the process was complete on one side, I simply flipped it over and repeated for the other.
loosened and removed the bolts, gripped one protruding end of the flatbar in the vice, and just kind of gave the receiver a little twist, and it popped right off.
I took my calipers, locked them on the measurement for the front of the receiver interior (1.170"). and scribed a line where the top rail will be bent, measuring from the inside bottom of the receiver. same for the rear interior receiver measurement (1.620").
I then took my flatbar, inserted it into the receiver, and C-clamped it on the scribed lines. I then put the receiver in the vice with a couple pieces of 3" angle iron laid over the vice jaws for support, and to make the jaws longer. Then evening all of this up , so that the angle iron and flatbar are flush, with the metal for the top rails sticking out, I cranked the vice down super-tight, and hammered with my block again, in the same fashion as I did the sides.
Next step is to find yourself a kit, build it, finish it, love it, and do it again!