I had:
/*
* These were used to fill up flash to debug some issues with
* optiboot uploads of large sketches.
*/
#define FLASH_STRING(name, str) PGM_P name = str;
#define make64bytes(a) FLASH_STRING(__flash1 ## a, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
#define make256bytes(a) make64bytes(x ## a); make64bytes(x1 ## a); \
make64bytes(x2 ## a); make64bytes(x3 ## a)
#define make1kbytes(a) make256bytes(x1 ## a); make256bytes(x2 ## a); \
make256bytes(x3 ## a); make256bytes(x4 ## a)
#define make4kbytes(a) make1kbytes(x1 ## a); make1kbytes(x2 ## a); \
make1kbytes(x3 ## a); make1kbytes(x4 ## a)
#define make16kbytes(a) make4kbytes(x1 ## a); make4kbytes(x2 ## a); \
make4kbytes(x3 ## a); make4kbytes(x4 ## a)
make16kbytes(foo);
make16kbytes(bar);
#if 0
make4kbytes(bar);
make256bytes(baz);
make256bytes(baz1);
make256bytes(baz2);
#endif