I have the T3x Varmint in 223.
Not being able to find a Bobsled single load mag adapter in Canada, I made one from a chunk of "white nylon", which is redneck speak for what is probably delrin.
It works very well.
No machine tools other than a drill press and a bandsaw. A hacksaw or table saw would have worked to cut it to the right thickness. Then shape and file it by hand. An auto body file works good for the rough work.
The trick is getting the angles just right, the concave "nose" needs to be angled slightly up to guide the round into the chamber while holding it centered in the mag. Larger calibres aren't as hard to single load, but the small 223 rounds want to butt their nose against the barrel at the front of the action. Irritating.
The raised front end simply gets pushed down into the mag as you close the bolt, then pops back up when you open the bolt, ready to load the next round.
These sit in the regular magazine in place of a cartridge, on top of the follower. If you want to use the mag normally, just pull the adapter out.
I've made these for CZ .22 mags as well, they work perfectly.
The idea came from a guy on RFC that sells them in the US for various .22 magazines. The Tikka 223 adapter is just a bigger version of the same thing.
I've also removed the ejector pin and spring from the 223. I simply use a fingertip to pick the empties off the bolt face. They stay clean and dry, plus the Tikka ejector spring is way stronger than it needs to be. It's strong enough to dent the case mouths when they hit the inside of the action when coming clear of the chamber.
If I needed the rifle to eject empties, I'd re-install the ejector with a lighter spring.