I would suggest you check out speedwell. Sorry, don't the URL handy (but you can google too)
The suspended gong style is the easiest to make. You might want to check you local metal fabrication outfits and see if they can do it and save the shipping cost. You'll need 3/8" T1 or equivalent. Don't use mild steel as you'll get some scary ricochets.
I have some 3/8" T1 targets and they work fine for pistol calibers. My friend's 454 Casull won't scratch, bend, or dimple the stuff (even at very close range). T1 is very tough stuff.
However, .223 rounds out of my AR-15 tear it up something fierce. At 50 yards, 55gr soft point bullets penetrate the steel after repeated hits. I recommend 1/2" or even 3/4" T1 for rifle fire inside of 100 yards.
Quality steel costs a lot and steel strong enough for rifle fire is even more expensive. I've found that MGM makes the best steel targets that I've found. I've shot two of their rifle targets (SOF & Lolli-Popper) and was favorably impressed. Their URL is http://www.mgmtargets.com I have a review of those two targets at my web site. The URL is http:/thecurmudgeon.freeservers.com/fa-index.html Look in the Gear Guide Section