Get a full/big tower, preferrably one with 1-2" of space on the side of the 5.25" units.
There are not many places to mount the motherboard but there are a few. What you can try to do is cut out a section on the back of the tower just big enough for the ports but not for the 2 outer screws for the monitorport and mouseport. Drill holes and the screws can go trough the holes to secure the motherboard against the back. This will hold it quite firm onto the back of the tower, and then you can use the 3 holes on the front of the motherboard (2 at the front and 1 close to the CPUslot/Mouseport) to mount it firmly to the tower (use spacers).
The turboboard shouldn't move around to much in the CPUslot so to secure it you could just use rubberbands (I know, sound amateurish but it works well and is flexible for any turboboard and simm configuration).
A AT->Amiga or ATX->Amiga power converter is pritty easy to create self, the leds are more or less strightforward and 2.5"->3.5" IDE cables are also quite easy to make yousrelf so the only real problem left is the keyboard. I sugest you buy an EZ-key solution or simular (allow to connect external Amiga or PC keyboard to the A1200).
If you're thinking about expanding the system later on you could try to prepare the motherboardmounting for a Mediator PCI bussboard or simular so that the PCI/Zorro slots are level with the backplates, but that is of cause optional.
