You did understand that you have to use both commands, right? Like
set(my,POLYGON);
c_setminmax(my);

Btw. if you are using A6 I think you have to wait one frame before calling c_setminmax because otherwise the values get overwritten in the first frame after creation, i.e. change

c_setminmax(my);
wait(1);
vec_set(my.min_x,vector(-22,-5,-20));
...
to

wait(1);
c_setminmax(my);
vec_set(my.min_x,vector(-22,-5,-20));