I've heard of guys with a Landon Bar (basically a loop that goes between the uprights with a contour to match the fender) but I believe they are difficult to find and pricey. Not sure about "caps".
As an alternative, why don't you just take leave the side brackets off? Either put the eyebrows on (they can be found on eBay etc. for not too much $$) or if you have saddlebags make some spacers. Might be a cleaner look depending on what you're after.