Custom/DIY Metal-Core PCB?
One of the best tutorials I find is Google Images.  That leads to:-



and aluminium PCB manufacturers

At first glance, it looks fairly simple for DIY, especially since you're making a one sided board.  The insulating layer automatically becomes the fibreglass or epoxy paper part of the stock PCB. Your dielectric won't be as thin as my image though and this will seriously compromise your thermal conductivity. 150 um is ~1/10th of the thickness of a 1.6mm PCB, and so thermal conductivity will also be ~1/10th. You will also have to allow for the thermal resistance of the adhesive, which I'll suggest that as a DIYer you won't be able to make particularly thin.

I expect that principal advantage of this form of construction comes from the very thin dielectric layer.  If you can't reduce this to a similar thickness, the DIY route seems implausible.  Although, flexible PCBs can be ordered and their flexible substrate  is very thin.  You'd still have the adhesive thickness issue though. 

Remember that the thermal source is the LEDs not the MOSFET (typically unless serious PWM). So do you actually need  a metal PCB?  Or can you simply build this thing traditionally and mount the LEDs with dielectric onto a heat sink? Or no dielectric - most high power LEDs already have provision for direct heat sink mounting.