Better way to keep this tension curtain rod from sagging in the middle? Tried using a sticker + ziptie but that didn’t work

So I have a very long window (8’+) feet and getting a curtain rod mounted is kind of a pain. I eventually opted for a tension rod, and it’s mostly great, but sags a bit in the middle—still functional but a bit annoying. The ceiling above hits metal pretty quickly so can’t really drill a mount above it. One solution I tried was using one of those wall-hanging stickers stuck to the ceiling, and then tying a ziptie to it to boost the curtain rods. But this clearly wasn’t strong enough.

Any ideas for other ways to support the curtain rod in the center so it doesn’t sag? I’m at my wit’s end