While I'm sceptical about modifying the stock intake components to increase fuel efficiency (as opposed to power), warmer (less dense) air isn't a deterrent to fuel efficiency. The LH system with an Air Mass Meter detects the mass (not volume) of air entering the system, so it's pretty much independent of temperature or barometric pressure. It 'knows' the density. It then will calculate and inject the proper amount of fuel to achieve chemically correct combustion, and the O2 sensor fine-tunes the process.

Cold, denser air can result in more power, as the mass of air drawn in is greater (more Oxygen molecules per piston intake stroke) and you can thus feed it more fuel. Bigger bangs. But the cold air generally will not vapourize the fuel as completely as a warm/hot airflow, so some of the fuel can remain unburnt (high HC readings in the exhaust). Getting full power out of a cold air intake means richening the mixture to give slight excess fuel so all the available O2 gets used in the reaction, compensating for the small amount of fuel that doesn't properly vapourize and get combusted. This is not the recipe for economy. That recipe is: warm intake air for complete fuel vapourization, and the leanest fuel mixture that will give proper combustion. This is the way the Volvo factory has it set up.

Probably the best "modification" you can make to the intake in the quest for more MPG's is to install a new air filter.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, 94-944 B230FD; hobbycar 77 MGB, and a few old motorcycles)