Have you tried the Raco 976 metal goof rings? I used them a few times in the past and they seemed to have a slightly smaller profile than the plastic extension rings. They work with one or two-gang switch boxes, one-gang handy boxes and one-gang plaster rings where the device is centered, so maybe they will work with two-gang plaster rings.
I know those plastic Arlington BE1 and BE2 rings generally work okay with two-gang plaster rings for depths greater than around 5/8" or so, but because they are tapered like a wedge, I find they usually donít fit well and the screw holes donít align properly with the plaster rings for shallower depths less than 5/8" without some cutting and modifications. If youíre using the two-gang BE-2, you might have better luck using two of the one-gang BE-1 extensions, as that might give you a little more wiggle room for adjustment.
Our inspector won't let us use those as they have no UL listing. I am just going to have to use the Arlington plastic ones and butcher them up. I had fair luck with cutting the corners out with a Dremmel Multi Max.
Thereís also these plastic receptXtenders. They are UL listed and come in both one and two gang with a different color for each depth size. Since these mount on the outside of the box, instead of inside like the Arlingtonís do, it seems like they should work better with the plaster rings. Supposedly these are available at HD, but you can also order direct from the manufacturer. I think these may be what Mike was referring to in his post earlier.
I have a couple issues with the arlinton BE Box extenders older metal boxes the extender sometimes wont pass the internal romex clamps and some older outlet boxes have machine screws extending inside the box that hold on a bracket like these
I remember the old BE-1ís had the corners slit so they could fold in as they went in the box and they seemed to work better with that style of gangable old work box than the newer BE-1ís with the solid tapered corners do. I think Arlington had to change the design a few years back to get them UL Listed. I know the old ones with the slit corners werenít UL listed. I think I may still actually have some of the old style ones on the shelf somewhere around here.