How can I build custom air ducts for an in-wall heater?
Probably not the answer you want to hear, but the manufacturer of your heater would most likely prohibit the alteration of air flow into and out of the unit. Doing so would void the industry safety certifications required for manufacturers, the warranty, and would probably not be safe.

Additionally, all wall heaters have minimum clearance requirements for the placement of objects in front, 8" is likely too close for placement of combustibles (like wood cabinets). 

I would recommend that you discontinue use of the wall heater if it does not fit your design plans. There are many types of plug-in electric room heaters that work for a variety of room sizes. Many of these are very quiet, inexpensive, and efficient (probably more efficient then that wall heater) and you can place it anywhere in the room you want.