Not the cheapest solution ($150 per rack)and a bit of care is required for real security, but it's hard to go past the Thule rack where you keep you wheels on !!!! Especially good for 20mm without QR
A nice lttle bar comes up and clamps the frame. I believe there's a special Freeride version now available which looks cheaper
BUGGER can't get the image in , just google Thule