I bought a yellow Fluidium replacement and moved it to an Astro.
Turns out that Fluidiums contain about twice as much wax, but exactly the same amount of fluid as an Astro. Sorta half confirms my feeling that Fluidiums simply are upside down Astros. ;-D
First I used both coils from the Fluidium, but it turned out that the small coil had to be removed and the large coil had to be widened. The gap between the two coils was too large and caused most of the wax to collect on the small coil, centering it in on one focal point thus causing overheating.
The large coil first wasn't wide enough, so the molten wax disconnected from one side of the bottle, causing a gap between wax and glass.
Now it works perfectly and is really nice to look at, here's some pics: