Metal markers made out of flashing would be one way to go.
I had some student make markers for our school garden and also wanted something large enough to be easily read. I wanted something that would last for years and withstand the elements. So I decided to go with metal. We tried a few different types and liked the copper flashing the best. I used this
copper flashing Its a bit expensive as it is copper but I really like the look and a roll will make hundreds of markers. Plus the softer copper is easier to write or press into. I also love the way copper weathers so weathering and the elements are actually a positive. If it becomes too weathered dark/hard to read a quick buffing will have it looking like new again.
Here are some of the finished markers the students made.
I've also used thin aluminum flashing for decorative projects around the garden. We had the students draw pics into squares of thin aluminum flashing and then tacked them onto the wooden raised beds. They've been out there for I think four years and still look good. So that is an option if you want less expensive.
You can just use a thick pen, large nail, or awl to write onto the flashing. Just want something that leaves a nice indent in the metal. The ones I had to students do we used decorative letter punches from the craft store so they are fancier. To make the letters more visible we spray painted them. Just lay out the markers and give them a quick coat from a metal spray paint like Rustoleum. Once the paint dries take some steel wool and rub the paint off from around the letters. The paint stays in the depressions and makes the wording darker. These are pretty thin so I think they'll hold up better attached to something solid. You could use wood as the backer. I plan to use plexiglass (because I live in a rain forest and everything stays really wet, molds, and rots here) so it lasts longer but it will ad to the expense. The copper flashing tape already has an adhesive backing so it'll be easy to attach. Finally I'm going to drill holes and use some metal wire to make stakes.
If I was making hundreds of these I'd probably attach the flashing to the backing (thin plexiglass or plywood) as whole sheets. Mark them into an even length x width and then hand write on them. Paint the whole thing and use a fine sanding disk or buffing pad to rub the paint off the entire panel at once. I'd probably go ahead and drill the holes while it was a larger sheet as well. Then I would saw them apart into individual markers along the established lengths.