There are methods, and I think this one is best.... I have worked on many jobs where there were very large plaster castings for them. Some as big as 4'... (2-3' are common to Victorian homes - that are common here.) In the old days the plaster'er? would cut slots for "keys' to fit in both the casting, and the keyways of the lathe. (The space between the slats) But - In most of the jobs I have seen them done in (I've not had to hang them myself) they gouged the back of the casting with a grinder, sloped plaster on it and then screwed it to a piece of plywood that has metal lathe screwed to it, then mounted the plywood to the framing between the joists. After that sets, they lathe up to that and float it into the edges.
If you have a line laser... Place the medalion on the floor on your CL in the orientation you want. Then shoot the laser up to the edges of the framing, and transfer the line. Or go ol'skool and plumb-bob it... them mount it to ply sticking out each side in the method above. Then mount that to a blocks between the joists. Solid and fast...
One thing I have always had a problem with, is that one of the local companies that makes them here does reproduction work of older designs - originally made to fit a GAS lamp. (a hole 1 1/2" to work in) So a 3/0 ring will not fit in it, or be so deep that screws wont reach the box above. Some are 6-7" thick,
like this one . So I make sure to be on site with the fixture so as to make sure the box can make it, and the fixture will cover what I force them to remove from the casting. Often I will need to do some severe head banging to get someone to grind out the back for a 4/0 box, with a 2" 4/0-3/0 mud ring with a crow-foot hickey inside mounted direct to the surface of the cieling framing, or even firred down more, or to the ply that goes between the joists.
Be wary though, the bigger the medalion, the bigger/heavier the fixture! A few years ago I had a customer throw me a loop, and had medalions installed, then showed me a 500 pound wrought iron fixture that needed to be hung with 1" all-thread. (They
stole/bought it from some church in Spain as a candle chandelier and had it wired.) It was a Polish joke install - 3 people to steady it on a panel lift, and one guy to get told to hurry up and wire it....
