I can't say it's a "good" idea, but it was an idea that's worked well for me for the past 6 years...
I bought one of the first RTV's back in 2004.  At the time, there were no forum discussions about the RTV.  I had a concern about the integrity of the sides of the cargo bed when I'd fill it with wood or stone.  I wasn't sure if they would bow out or get banged up badly.
So, right after I got the RTV, I removed the OEM bed sides.  I then made my own cargo box out of pressure treated lumber as I knew the wood could take the impacts and if it did get all banged up, I wouldn't care as I didn't have much invested.
I laid some plywood on the bed floor then built sides out of PT 5/4 decking boards with 2x4" vertical supports.  The top 5/4 decking board has some 2/4 blocks to keep it from sliding around but is otherwise not secured.  I lift it off, and there's storage under there.  There's a bit over 3000 cu/in of storage which is the equivalent of 2 medium sized backpacks.
Once I stained it, it doesn't look bad.
Here's some pics.  BTW, in the unstained pics, there is no tailgate mounted yet.  There is one there, made out of a 2x12.
A hood rack (if they make one for the 500) is another idea.  I have one but don't use it as much, since I can put things in the back.
I also added some hooks to the ROPS for holding things.  The single J-hook pictured below is mounted near the top of the ROPS.  I use that for a coat hook.
In another picture below, you'll see some brackets coming off the ROPS with an eye-bolt coming off them.  I have one bracket on the front ROPS, another on the rear.  I use this setup for carrying long items.  I carry extension ladders by just setting them on top of these brackets.  For other big items, I carry a couple small chains (like safety chains from a trailer) that I wrap around whatever needs carrying then hook them on the 2 brackets and off I go.