I pulled my Idler pulley and got all the dimensions(I think) to do the drawing. I found some scraps that worked out quite well for giving you an idea of how the strap mounts. This is 3/8 aluminum stock, so it is a little thick. But you can see how it will fit and there is plenty of room for the strap. One pic shows the mounting points(circled in black) and then a couple of pics of a mocked up strap with the pulley.
The pulley uses a 10mm bolt, the unused mount we found is a 6mm bolt. The spacing is 3.780",center to center. The idler bearing sits about .475" lower than our strap so that would be what we need for a spacer/seat from out strap. Anyway, here is what it could look like...

MikeR


edit: here is the first pass drawing of the strap and how it fits.