A key factor in choosing between either a solid MAF pipe or flexible MAF hose is if any component of the air intake system is rigidly attached to the car/frame on the opposite end from the turbo.
For example, if the factory stock air cleaner box (attached to the frame) and MAF sensor are used in conjunction with a hard MAF pipe (replacing the factory FlexFab hose), vibrations from the engine and accessories will be transmitted thru that hard connection to the MAF. The electronics and calibration of the MAF will be damaged over time. That is why the FlexFab hose was chosen by design to limit the transfer of vibrations to the MAF.
Many have eliminated the stock air box and run just a hard MAF pipe connected to a MAF sensor connected to a cone filter. This assembly basically floats as it is only a hard connection at the turbo inlet - the end of the filter is not rigidly attached to anything. Depending on the length of the MAF pipe, some have made a hanger of sorts to support it since the weight of the filter, MAF sensor and the pipe itself act like a cantilever at the coupling at the turbo inlet.
Some may remember Pete Tomka and his cold air intake system. The lengths of flexible hose (3", 4", etc.) were CEET aircraft hose - specifically it was Aeroduct brand.
Some may remember the KenneBell cold air intake system whereas flexible hose directed air from an opening in the driver's side air dam to the enclosed cone filter.
So using a flexible MAF hose is not a problem. be sure to use the CEET or CEET equivalent hose. I'm not a fan of the aftermarket stock replacement hose - the corrugations are too pronounced.
@grass doctor has already covered the best sources for aircraft ducting in the post above.