Tested the first prototype of this design (external 12v brick), and it worked - booted my LC III. Barrel jack input (2.5mm center pin) on the left, 12 to 5 buck chip and an inverter hanging off the 5v to get -5. Fuses and protection zeners built in, to safeguard your Mac.
The final design might be a two-PCB system. One small board will sit at the back and hold the barrel jack input and power switch (this prototype PCB has through-holes for a switch but was jumpered to be always on). That smaller power-in board will connect to this main board. Would allow a more normal look, with switch at the back.
This design can be cheaper than the previous ones (Around $32 for a kit), for one simple reason: The 12v supply is external, not integrated, and the $30-ish kit doesn't include the 12v supply. That 12v supply can be had for around $13 to $20 depending on how nice you want to go.
So really it's not much cheaper than the previous design, as the total for a kit climbs close to $45 when the power supply is included. But this design has one small advantage, for the case where you have several LCs and only use one at a time. The external 12v brick can easily be swapped between LCs that use this internal board.
I've been unable to find ways to lower the price further, and it's rather frustrating. Even this design, using a cheaper and more available buck regulator ends up costing just a few dollars less overall.