How to model a one-port (reflection) measurement in SPICE?
Simply use an AC simulation to find the input impedance of your 1-port, then calculate the reflection coefficient (\$\Gamma\$ aka \$S_{11}\$) from the input impedance

$$\Gamma = \frac{Z_L-Z_0}{Z_L+Z_0}.$$

To find the input impedance, just excite your 1-port with a 1-A ideal source. Then the voltage you read across the terminals is numerically equal to the input impedance.