RTK GPS altitude reference mismatch

I’m currently using the Holybro GPS as well as a SwiftNav RTK GPS on my airplane. The Holybro GPS outputs altitude in MSL, whereas the SwiftNav RTK base station outputs altitude in Earth ellipsoidal altitude. I have contacted SwiftNav and they do not support the conversion of ellipsoidal altitude to MSL.

Is there a way I can convert one altitude to the other reference frame?