Use a sharpie and a rock.
The ones I made in January held up well through the wet part of winter so I'm assuming these will easily last through the dry summer (or at the very, very least until the plants start fruiting, negating the need for labels).
I'm currently building a climbing structure (as Biscuit called it) for vegetables. It's made of the branches of my dying cherry tree (boo hoo). Hoping to get most of the rest of the garden planted today before the seedlings ALL die in the heat.