Here is explained the modification to choose a degree D=513 instead of D=512.
In all files config_HFE.h, just replace

/* Degree of HFE polynom : Deg = 2^DegI + 2^DegJ 
 * or Deg = 2^0 (with DegI==DegJ==0) */
#define HFEDeg 512U
#define HFEDegI 8U
/* Requirement : DegI>=DegJ */
#define HFEDegJ 8U

by:

/* Degree of HFE polynom : Deg = 2^DegI + 2^DegJ 
 * or Deg = 2^0 (with DegI==DegJ==0) */
#define HFEDeg 513U
#define HFEDegI 9U
/* Requirement : DegI>=DegJ */
#define HFEDegJ 0U