@Canuck, I thought about building something, but in the end I bought 2 vertical planters and they've worked out perfectly. The planters are made of fabric so excess water can drain through and we added wooden supports at the bottom so it wouldn't topple. I have around 16 lettuce plants, a tomato plant (with seven little tomatoes growing and many more flowers), some coriander and basil. I bought plugs as I wasn't confident with growing them from seeds, since I've never grown anything before. Here is a picture from the day I planted them at the end of May and a picture I took today. The lower planters are empty because a neighbourhood cat was digging up the small lettuces, but I'll plant some other larger veg/herbs there soon.
I don't know much about how long you should let lettuces grow, but I'm planning on harvesting them today!
Here's pictures of when I planted them on May 27th and today:
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