This is to confirm that the Aircraft Spruce Van's retro fit locking gas cap (05-01006) does fit the RV-12 filler neck. I haven't pro sealed it in yet but it appears to be a perfect fit. Once Hal gets the one that Van's sells we should have all the data. That one may be this same one and if so you can buy it from Van's for a few dollars less.
The cap is nicely made (in England), operates smoothly with the key.
The opening will be smaller as noted previously and the cap is non-vented so on the RV-12 you will have to have the vent mod installed.