I could not find a string spool holder that I liked, so I made my own.  The model was intended to allow a spool of line commonly available from Home Depot to slide over the core of this model.  Then the two halves of the model can screw together to hold this spool of line in place.  The blue model shows the spool installed on the part.  

 I like to hang my tomato vines from nylon string.  I make a loop and attach a hook that fits nicely over a 3/8 (10mm) inch bit of rebar.  The hook would probably work well with cow panels as well.  It fits nicely over the plastic core of a dog waste bag spool.  I added a slot in the string holder so that I could put the hooks over the plastic core and then wind the string sections up for tangle-resistant storage for the end of the season.  The black model shows how this works.

I printed both the blue and the black spool holders in PETG.  Supports are required, and a bit messy.

This project is incomplete.  I intended to add a kind of holder and also a hand crank, but the holder is working well for me now.  The open scad file is highly parameterized.  I have not made it especially user friendly though.  The comments may be descriptive enough. You might need to play with the settings a bit to figure out what they do.  The attached models use ¼-20 bolts and nuts, because I have a lot of them for some reason.  You can change the bolt size in the SCAD file if you cannot get your government to move away from the metric system.  I like to embed the nuts in the part, so you have to create a pause in the slicer so you can put the nut in place.  I really need to add clearance to the bolt side hex.  It can be tight.  I had to use a big drill bit to create a clearance hole for the bolt.  It doesn't need to be a tight hex all the way down to the end.  The hex part only needs to hold the bolt head in place. I hope folks find this design as useful as I have.  I may even finish it one day.