I'll go take some pictures and upload in a moment. I use a P clamp (with rubberized coating) on that chainstay brace, but didn't need a lot of spacers. I don't know how big of a spacer/spacers will work and stay rigid, but it's easy and cheap to try different things.
The other option - which might effectively be what I did - is to let that chainstay brace dictate the position of the fender, and then match it at the other mounting points. In my case, that meant notching the fender at the bottom of the seatstay brace and bottom of the fork crown. A bit of hassle, but it's also what allowed me to fit 2.1 and 2.3 tires. (I've since decided I like sometĥing slightly smaller, but it's nice to have the option.)
Because every All Rounder was essentially a one-off, built-to-order, custom, it is likely that the conditions on your bike won't match mine or anybody elses, so you're going to have to figure it out through trial and error to some degree. Because I like fenders, I feel like vertical clearance is too limiting on every Riv I own, but this one is particularly so. I think the seatstay brace (and crown) is located at a height that would allow the use of caliper brakes, even though it also has canti mounts. Kind of emphasizing it's versatility, but with reduced clearance that makes it not so easy to utilize modern, big-volume tires.