Hmmm.
Had time today to look for the brush bar drawing on REMLR but couldn't find it either. However, I had a look through my own files and found a pdf drawn by myself and most probably copied from the original, because it describes unobtainable steel sizes, such as, 48 x 10mm ( 1 7/8" x 3/8").
If you use metric steel, such as 50 x 10 flat, it looks a bit too chunky but I suppose it is better than nothing. One thing I remember, is that the fillet welds naturally enough shrink the flat bar. If you don't have a good clamping-jig, this can be countered by laying a bead on the other side of the bar to shrink the metal the other way and then grind the bead off later.
Drawing attached,
Cheers Charlie