You ask "what have other people done" and, unfortunantly for you, I'm the first to respond. Unfortunantly, because I had to goggle 'riser bar' to find out what you were dealing with. Having done that, I fail to see why a pair of bar ends would not solve your problem if your bars are wide. They come in many shapes. Just pick the most likely and try it. Return if it doesn't work. Otherwise you'll have to change bars to get the hand positions.

Of course, if you have padded grips of some sort, rather than tape, that would present an inconvient mounting problem for the bar ends.

Another somewhat radical option would be aero bars. They might be mountable on the risers. But I'm just speculating in left field, trying to give you ideas, having never used risers, bar ends, or aero bars. I get my hand positioins from a bull horn with friction shifters and interrupter brakes. Wouldn't trade.

Your lack of response is probably because it would be very unusual for a touring cyclist to use riser bars. Bull horns either for that matter.