Small-signal control-to-output transfer function for SEPIC-Cuk converter in open loop

Hello,

How to get Small-signal control-to-output transfer function for SEPIC-Cuk converter in open loop? As an example I used AN-1106. But I am failed to calculate Vref/Vout plot in LTspice (see attached).

LT3759_DC1787_SEPIC_CUK_TRANFER_FUNCTION.asc

How could I get an average model for SEPIC/Cuk converter?