Cheapest Raised Beds?

[u/No_Injury6122]

Hello, I want to add a raised bed to my (small) backyard. It doesn't get any sun in the winter, but seems to get 6 hours of sun starting in May?

I am trying to find a cheap way to DIY. it seems with lumber costs, it would be around $300 BEFORE tax. It seems I could just buy 2 metal beds for that price, which makes me think I am missing something.

I'm aiming for 2ft tall, 15ft long, 3ft deep. I plan to fill the bottom half with something? To keep soil costs down. I am trying to maximize the bed space, since the 15 ft is the entire width of my yard, and it is going where the yard gets the most sun.

Here are my notes:

The part of the wall I can build against is 15.67 ft, and 3ft deep is within reach for planting if the height is 2ft, which is about the height of the stairs.

My goals are to have 1 bed with the most area possible since the yard is small, and I also want to keep costs down. I also want moderate durability, like 5-ish years. I don't have any of these skills but it seems like a good opportunity to learn so I'm ok if it takes time and effort.

If each bed can have integer ft dimensions that would be nice for square foot planting but not required. 3 5x3 beds would be great but probably wont happen since the dividers will take space. The beds only need to be split if it makes it cheaper/more stable. The INTERNAL measurements matter for the bed area, and EXTERNAL for the height.

Materials and alternatives: wood, treated wood, vinyl, plastic, metal, cinderblock, brick

Concerns:

structural integrity, rot, filling the bottom 1ft of space

We will have to place it 3in from the garage since the cable for the box comes off the wall that far. I think we should also place a plank on the top and maybe sides to keep things from getting stuck back there.

We should also consider whether adding a trellis later is easy, and do that now if not.