On mine, I bought the longest grade 8 bolts I could find, and drilled and tapped the hole that is already in your frame, just in front of where they're mounted on the engine (the square indentation with the hole in the middle of it. I think a few companies make sliders for the 919 in this location, but I just retrofitted mine.
You may want to add spacers to make them stick out far enough to be effective. For this, you can either have a machine shop spin up some quick spacers on the lathe, or just use washers/aluminum stock/whatever you can find.
Let me know if you have questions. I can post/send pics if needed if you want to show a machine shop what you want.
2002 Honda 919: Erion Racing slip-ons, Givi bags/windsheild, Napoleon bar end mirrors, Pro Taper 1 1/8 bars, sliders, etc
1994 Suzuki GS500e: Completely redone suspension, V&H exhaust, fun little bike