String - but thick, tuff, synthetic

Also used wood once but it was a pain to work around.

Initially, I used vinyl window blind slats & hand watered.

The next year I used them again & hand watering became too much during our summer drought. The beds really do dry out quickly, so also used a sprinkler on top of a bucket.

The 3rd year added soaker hoses & didn't put the slats back in place. I lay them out a bit when planting, but remove them.
I dislike the appearance of the grids & don't really need them anymore to plant.

I learned to space with my hands & if a bit more space is given I need to water less. If plants are more productive even though less plants in the space than Mel's suggestion my yield is the same because each plant gives more.

I tried the venetian blind slats but those got blown around on a windy day. Tried green garden twine but that disintegrated within the season. Finally used nylon tomato string and it worked great. Screwed small screws on the top of the boards and wrapped the twine back and forth from side to side.
Note: Tried screws on the inside but the string kept popping off.