remember chocolate doesn't melt at room temp if properly tempered (or could cheat and use candy melts.
as for tiers ---
take regular cake pans and use them as giant chocolate molds -- so pour in the choco, swirl around to coat whole pan, pop it fridge to harden up. May have to repeat a time or two to build up thick enough shell
while still cold, should be able to pop out of cake pan.
to even up the edge -- heat a cookie sheet and run shell across it to melt edge
then stack gluing together with melted chocolate as glue.
would stack working upside down so that could have the goodies inside.
then turn right side up and have at it with decos.
could also buy pans like these:
and just make a one piece shell as described above.
just standard chocolate shell making using a different type of mold.