The principal criteria are pressure drop, temperature and sealing. The device has to live in a very hot atmosphere and survive to work the next time. You don't want it constructed so that it adds back pressure to the turbine and you don't want it leaking either to the atmosphere when in straight through mode, nor into the HRSGS when in bypass mode.
Another criterion worth mentioning is actuation. Pay attention to that. There is a good possibility for unequal expansion, corrosion, and all kinds of ills that will quickly overcome a cheap actuator if you skimp in that area.
rmw