Interesting coincidence. I just built a preheater exactly like this and had planned a write up. So far, I have used it only once for test and once before a flight. It seems to work great.
Basically, buy this heater +/- the batteries if you want cordless. Grab a 6" (I think?) to 4" HVAC reducer... one that will fit over the green outer tube on the heater. In the same Home Depot isle as the HVAC reducer, get some of those small HVAC style self tapping screws. Screw the HVAC reducer to the heater, one screw every 45 degrees. You will see where the reducer contacts the outer heater tube best, screw in there. Then, just use a 4" dryer line. Get the hard stretchy solid aluminum one, not the foil one. Stick that thing up in your cowl flap and you are good to go!
I scrapped a few bucks trying a T piece with two lines and some other experimental parts that ended up not being necessary. Keep it simple like above. The weak link now is the dryer tubing. I think it would actually break after some use. So the options are to either just replace the dryer tubing occasional because it's cheap or maybe get some 4" SCAT.
Edit: I'm thinking the reducer may actually be 8" to 4". Test it in the store when you buy the heater or measure it at home.
