I wouldn't be too concerned with finding a 'weber' grate to use as a raised rack --- find whatever brand that is the 'right size' - whether single or double - that will work for what you want to do with it. There are many of other grill brands that have some weird sized grates that could work really well. I wouldn't even be concerned if it wasn't SS myself.... it will last if its easy to clean AND you actually clean it.
This is the cooling rack that gets used regularly on the Spirit --- its really nice that the 'feet' hang nicely on the grills warming rack and the cans to keep it from sliding too far front to back.
.
View attachment 106491View attachment 106492
I wish I had two of them instead of one.... nice and sturdy and easy to clean. I can't remember where I picked it up some 25 odd years ago.
A good search of the usual place may turn up similar racks with the desired dimensions and features that would work really well.
Heres a nice contender that is worth looking at....
I haven't taken any measurements for the distance between the rack/roti notches, but 10" I would guess would be pretty close.
There is nothing wrong with leaving the stock warming rack in there too to help bridge the gap to the roti notch.... then all you need is a bar for the roti notch --- pretty easy.
These look like they would be more 'sink friendly' for easy cleaning..... Should work well if you keep the stock rack.