I built a ATV Heater out of some flexible dryer hose.
What I did was simply, attached the dryer hose to the back side of the radiator.
I then ran the hose to a "Y" and then on to the foot wells.
It was a very tight fit to get things to work out.
The end of the hose towards the radiator I have a 2" to 3" adapter. I put it on to gather more air.
The adapter is zip tied to a bracket close to the radiator to hold things in place.
I also have a override switch on my radiator fan so I can turn the fan on.
It doesn't put out a huge amount of heat, but it does help.
It's on a Polaris.