for the arm for the row marker, not sure if i would go with a pipe or a rod, but more likely a square tube or rectangular tube shape metal. less likely to bend.
your going to want something heavy enough that it actually marks / dents / actually puts a grove in the dirt to mark your next path. a disc, like a 3pt hitch disc. get one of them circle disc and toss on end of the square tubing. with something. the disc will not hip hop and jump all around, but rather when a clod of dirt shows up in front of it, or a hole. the disc will run over it. vs getting hung up.
i suppose, you could do a "ski" or "shoe" vs a disc. but more likely wobble back and forth and not have a true distance. along the entire path as it might get hung up on things.
------------
i guess you could always do the "soap bubble" you can pick up a fairly cheap small little 12v electrical pump, from grainger, northertools, and the like websites/companys, feed the intake hose into some sort of "suds" liquid, and then just dangle a the hose on end of some rod that sticks out. but with that you really do not get a real good mark, but just general location. and you have to eye ball things.
-------------
you will most likely find it easier having a marker on both sides, vs trying to figure out were to mount a marker so it swings to both sides and have it in easy reach.
------------
it has been way to long since i been metal shopping, but normally menards, lowes, homedepot, have a small section, were you can pick out different size pieces of metal (rod, L, square) you might try picking up some that has a bunch of holes already drilled into the metal. so you can just use a couple bolts and some "butterfly nuts" so you can quickly adjust distance you want, without getting out a bunch of tools.
i suppose you might be able to just use a 2x4 as the main arm. and goto local hardware store and pickup some sort of "deck hanger" that is U shape. to use as a hinge point, drill a 3 holes in tiller, use some bolts to fasten the "deck hanger" to the tiller. and then drill a hole into end of a 2x4, so you can place a bolt through the hanger => 2x4 => and out the other side of the deck hanger. again ya most likely going to want some sort of wheel or disc on the end. so ya don't look back and see the marker all bent back and out of shape.
might advise making spot so you can add weight to the marker.
-------------
eye bolts can be used to "thread" the rope around things, if it is to heavy, use a block and tackle. it will require more rope to pull, but reduce amount of weight you have to pull to lift the markers.
