Well ok, this isn't really a mod, but I found a tankring from SW-Motech that fits the RX3. Now I can move my tankbag from the Vstrom to the RX3 and back.
I really like tankring tank bags. They don't scratch the paint like magnetic or strap on bags and unclip with a quick release. All you do is bolt on a mounting plate to the tankring and the bag snaps on to that. A quick wire pull releases the bag. The downside is the high cost compared to other types of tank bags.
My SW-Motech tank bag is called the 'Engage' ... AKA ... the 'Non-Electric QUICK-LOCK EVO Engage Tankbag'. It has a 7 liter capacity and can easily carry 2 quarts of oil or 2 full size water bottles with room to spare. There are side pockets, a waterproof cover, a carry handle and attachment points for accessories like a map case.
I don't like the gap under the curve, but that's as far forward as you can mount it. The Daypack or Micro models would probably fit better, but they're also smaller.
You can buy the
SW-Motech 'Engage' tankbag with tankring here for $239. Just be sure to select the Type 115 tankring.