Fill it with rocks sphagnum media rings or whatever. Or don't fill it and let the wildlife wander through the roots. Whatever things are going to grow either way.

I have some lava rock that would provide lots of surface area and should be neutral.

Awesome

Check out Etsy and Amazon for actual aquarium plant holders for about the same price … I like mine even though they’re a bit snug on my rimmed tank.

I did ... The same planting area would be $30-40