Gerry
see attached drawing...please verify against your top plates for accuracy (i.e. don't necessarily trust my figures). Also suggest you take actual measurements for your tube sockets and large parts as I had to do some filing with mine to account for part variation...easy when working with AL...more time consuming when working with 14G stainless steel. Likewise, the slightly thicker sheet stock means that you may be right on the margin for screw length in some cases.
All this said, the process on Stainless supply's website isn't terrible.
I simply traced the top plate onto a piece of paper, then measured the original AL top plate with a set of calipers, marking off my traced drawing as I went to avoid missing any critical dimensions. I used this to input figures into their web form.
NOTE: At the time, I thought I would save some $ by having them laser-cut the larger apertures and then drill the smaller holes myself on a drill press. That is what is reflected on the attached drawing. In hindsight, I would have them do everything and save the incremental effort.
good luck.
bryan