Secret Weapon Miniatures and I tried them out.
The adapters come in two parts. Each half is made so they line up properly with a post and hole arrangement. These are made out of resin, so super glue is necessary for them.
Here's a Space Marine Veteran I am working on. As you can see the two halves fit around the base. It's not a super snug fit, but it leaves enough space that a little super glue along the seams allows it to flow in. These would be much easier to assemble with gel type super glue rather than the thin type, unfortunately I am out. I had a couple of failed first attempts before getting it right. Fortunately I didn't glue any to the table top. :)
The one knock against it is that -- what may be difficult to see in the picture -- more often than not, the hole is full of resin. Of the three I have done so far in testing it out, two of them had filled holes on one side. One I was able to dig out, the other I had to carve off the post from both halves to get it to fit.
EDIT: After putting 30 of these together, I have unfortunately run into some other issues. Namely that there's inconsistency in their size. You can have several that fit snug to the base and then you will have several where you have a sizable gap between the base and the extender. It's not a deal breaker, but that gap is sometimes pretty big and requires filling.
Here are the three finished models. I have seen several adapters that just expand the base out and are just basically a round ring that the base sits in. What makes these nice is they are beveled like the original bases and are designed to fit around them and truly make it look like the original style base. I did manage to find one other company making a similar style base and making it out of injection molded plastic no less.The difference is in price. Secret Weapon offers them for $5 for 10 bases (20 halves). The other company offered them as $3 for 4 bases (8 halves). Granted, the injection molding is more expensive, but a little liquid green stuff and priming and these will have a seamless look. The others have the nice selling point of going together with standard plastic glue. I even posted a Kickstarter for a campaign out of the Netherlands for similar plastic adapter rings. Sadly that campaign failed, although they supposedly going ahead with production anyway.
One tip: You do need to make sure that the model itself is sitting flat when you glue the ring onto it, because it's loose enough that the model can wiggle around. This means that if it sets wrong, you can have a wobbly model.
I bought enough to do 50 models. I have a whole mess of Marines that I have stripped that these are going to go on. I am unlikely to do the ones that I have completed just for the issue of priming the adapters with a completed model on it. If I find a good enough method to do so with the ones I have done above I may do them in the future.