For at least 15 years, my writing and consulting partner David W. Spitzer and I have written positively about the Vortab line of flow conditioners from FCI. These unique devices are the only devices that can take a 100 upstream diameter flow upset location and turn it into a 5 to 7 upstream diameter non upset installation. They are patented, and they work.
Now, Vortab has produced something new, and very cool. They've produced an "insertion panel" design that they claim will reduce the straight run after a 90-degree ell to 1.5 diameters to 6 diameters after the elbow to the panel, and 3 diameters from the panel to the device...a maximum of 9 diameters upstream straight run between an elbow and a mass flowmeter.
This is significantly cool, patent pending, and available in 2" to 40" sizes standard, with larger sizes on request.
It is always a problem doing flow measurement when there simply isn't enough real estate to do it properly-and frankly, lots of mechanical engineers who do piping design are far more interested in how nice the piping looks, how symmetrical, how clean, rather than whether the flowmeter will work or not.
I don't do direct endorsements for products much, but Vortab has always been a favorite of mine for making troublesome flow applications work, and this version is doing the same thing. This is clearly a better idea than the typical flow straightener made of a plate with holes in it. If Vortab's claims, shown in the picture here are to be believed, and I think they likely are conservative, this device will work substantially better than flow straighteners.
If you want real accuracy in a gas mass flow application, you could build the Vortab plate and the flow meter into a spool section, and calibrate the spool as an entire assembly. That would be a good choice, especially for new construction.