This piece fits in the hollow rails of my Palram "Nature Greenline" greenhouse (may be similar to their Snap & Grow line). It's designed to insert, then twist 90 degrees and lock in place.

Hoping to use string hanging from a couple of these to provide a trellis for peas growing up from our aquaponics setup.

Note: my limited tests indicate it probably can't hold the weight of, say, a hanging plant due to it slipping in the channel. The current hole size is only suitable for string anyway.