Line it with ground cover fabric and you don't have to care about material, rot or anything ;)

I built a wood planter on legs with no bottom, attached ground cover fabric in a sling/U-shape that hung a bit away from the wood in all sides.

A dm of LECA, then soil and compost as per usual. LECA makes for nice drainage and water reservoir.

Keeping the soil away from the wood made it so I've had no rot or degradation at all for 6 years as air comes in around to dry it all out.

Sizewise I covered the whole short end, and made it deep enough to fit the door and other furniture, about 40x60x195 cm.

I have multiple different hydroponic setups on my balcony, and two ones with soil and permaculture.

The hydro ones are for annual plants (cannabis, herbs, tomatoes, etc.) and the soil is for the ones that will come again next year.

The soil ones are left for "nature" to take over. They are only fertilized when needed (organically of course), and otherwise left alone, because I don't wanna disturb the ecosystem that built up there.
There's constantly plant matter added, so there are always fresh nutrients available.

For the hydro setup, I chose a few DWC buckets (nutrient solution with air pump), a few kratky bottles, and some wick-kratky-hybrids.

They work fine and are mostly set and forget. The DWC needs some checking from time to time, but the passive ones are just getting refilled when needed.
They are all very lightweight (important for balcony), get no weeds, are very adjustable and flexible, the plants grow extremely fast, and much more!

How I built the passive wick system:

• Two planters are needed. One for the outside, one small for the inside
• Pull a polyester towel through it, it acts as a wick, pulling up nutrient solution from the reservoir
• Fill it with LECA or lava stone. They suck it up and store the water/ nutrients
• The roots will colonize the substrate first, and then grow to the bottom, like in a kratky setup

The DWC looks like that:

And here's the soil one:

And, last but not least, my Kratky army, made out of old bottles I found: