Hi Brian,

Indeed if you characterize the impedance you can arrive at a fixed matching network.

If we suppose the QCX requires a 50 Ω load, a reasonable assumption, and the QRP Guys tuner got it matched, you can try the following approach:

Disconnect the tuner and put a "dummy" load of 50 Ω on the _transmitter_ side;
Then measure with nanoVNA the impedance "seen" in the load side of the tuner, note its value.

The input impedance of the 50 W amp will be the conjugate of this measurement, so if you read, for example, 12 resistive and 40 inductive ohms, 12 +j40 Ω, then the input would be 12 -j40 Ω, i.e., 12 resistive and 40 capacitive.

From these numbers you can use the formulas appropriate for the match.

Depending on the values found maybe even a single L network could get the job done, as you intend to use a fixed one.

More details, as the frequency range of the intended match and the real numbers may steer the approach to other designs, but the road map is this.

HTH

--
Cesar, PY2CSH