Unless you enjoy doing a bit of cutting, fabricating and welding the happy trails crash bars don't play well with the rumbux lowers.
I had the happy trails already when I wanted to add a skid plate. The rumbux was the top of my list for a skid plate, so I figured I'd see if it worked well with the Happy Trails crash bars.
The rumbux and happy trails both use the same forward mount points on the side of the bike. A friend with a welder and some creativity helped me get the happy trails mounted along with the rumbux.
The right side wasn't too bad. All we had to do was stack the happy trails mount tab on top of the mount tab for the rumbux. For this, we cut the happy trails mount tab off, shorted it up the width of the mount tab and welded back on.
The left side was a mess though. They both mount to the tab under the radiator and the rumbux tubing was sticking out so I couldn't stack the mount tabs. We had to cut and reroute the happy trail tubing round the rumbux. While we were at it, I noticed that the piece that holds the lower radiator was gone so we welded a chunk of pipe to hold the lower radiator also.
I'm happy with the results, but if I were to do it over again I would purchase the full rumbux setup. I have some pretty stout protection for the bike with this combo now though.