Having carried those benches Charles mentioned, I can attest to them being nice and heavy. I have a bench much like those, and even donated a nice set of legs to the VA club when I left mine at the Nats one year. I used the same arrangement for legs that they did with the two pieces of telescoping steel pipe. I like the extra smooth stuff the best but I don't think it really matters. The biggest thing I'd say is to use a good steel square tubing for the table frame and then weld the floor flanges to that so you can attac/remove the legs. Splay them as Charles says, and use something solid, like pieces of 3/8x3/4" steel for the spacer that holds it at an angle. Weld them solid enough and you'll have a bench that you might actually prefer shooting from after you get used to it. Be certain to make it long enough for your longest gun, plus about 8" (is my advice). My heavy gun rests use every inch of the ones at the aforementioned clubs.
I'd also recommend welding big heavy washers on for feet, or if you happen to have some slugs from punching steel, use those with no holes in the center. A nice flat foot on the thing is a whole lot better in wet soil than having it sink into the dirt as you shoot. Ymmv.