this is the best method so far with what you have. I would not get that two flute end mill anywhere near a drill press of any kind, it will grab like crazy. you really need the counterbore drill, or just pay somebody to do it. How many holes do you have?
I just bought a similar one off craigslist today, the paint on mine is nearly gone. Is that the original color? It looks more blue than mine, but that could just be the picture.
You might check your area for steel suppliers that specialize in Tool and Die steels. most all of them will cut to length and often have leftover pieces that might work for the home hobbyist.