It would be nice, if you could add support for end rods, so one can place them on top and bellow slaps as well.

Sure thing, will look into implementing this over the weekend some time.

I just realized that this isn't as simple as I originally thought, because you can stack end rods on top of each other. So you probably need to "move" the whole stack...

Just want to clean this up a bit but the spaghetti code is in place.

End rod support has been added into the latest version.