There is no doubt a warm air mod helps the Honda Insight's mileage. However, the majority of mods I've seen send the duct work around the front of the engine or duct air from the cat under the car requiring long duct work. I decided to try a different approach by using the first cat behind the engine and just below the exhaust manifold.
I was lucky to find 2 pieces from my vintage VW parts bin that work perfectly. The first piece is a 2" by 24" fresh air hose which appears to be made of a dense paper with a metallic inner lining. These are commonly used on VW cars from the 1960s. A VW of this era uses two 12 inch length hoses but I was lucky to find one that I had not cut in half so it remains 24" in length and was the perfect fit. The second piece is a 2" exhaust coupler that attaches the hose to the intake elbow.
I removed the original intake and flipped the intake elbow backwards and positioned the rear under the battery tray so it faces the rear of the engine. I connected the elbow and hose with the exhaust coupler. I ran the 2 inch hose under the batter tray and placed it between the engine block and exhaust manifold behind the first Cat. It stayed in place by itself and needed no attachment point. I placed a zip lock bag secured with a rubber band over the hole for the resonator. This will allow it to be easily removed when outside temps get too hot to allow for cooler air to be pulled in. I monitor it all with a ScanGauge II. I would like to find some caps to use instead of the plastic.
I like this option because it allows better access to the front of the engine so I can easily remove the oil filter from the top.
I've had this mod installed for a little over 6,000 miles and it has shown no problems with placement or movement. The temps here this time of year have varied between low 30s to high 80s and I am seeing Intake Air Temps from 98 to 130. Many feel that 90 degrees is the optimum temp but my car seems to run best when the intake temps are 110-112 with water temps of 193-197.
I'm sure you could prepare some type of enclosure around the first Cat to produce even warmer temps if you are in a colder climate. The temps in January were reaching the low 20s and the intake air temp was rarely exceeding 100 so I decided to stuff some insulation (aluminum foil) around the open sides of the cat which has kept the temperatures more stable around 109-121.
I've attached pictures that I think are self explanatory but feel free to email me should you have any questions.