I could give you some suggestions for modifying the crop marks after making them with the crop marks filter in Illustrator, but what are you trying to accomplish with this?
In the example you posted the marks are very close to the object, but they remain relatively large, taking up space, and the objects are far apart. So this tells us that your purpose here is not to fit more objects in a given area.
Please explain what your goal is with this question.
Al