I used milk jugs and salad containers for a long time until it just got too messy having that many containers. I now use a low tunnel made from thick painters plastic and pvc pipe. Sometimes I plant directly in the ground or but mostly in well waterer pots. I think it takes less time to set up and can be dual purposes and used to extend crops in the fall. You just need to be careful when you plant...My first year all my tomatoes ended up sprouting in December and died.

I would think the larger posts could be used fine. Maybe cover the top with a large zip top bag and rubber band it down. I think generally you can use anything you can get soil at least 3 inches deep into. When I did this several years ago I didn't like dealing with the top of the milk cartons so just cut them in half and after watering and planting slid the bottom into a large zipper bag. Everyone has their own favorite things to use.

Thank you both for the input!

Perhaps I'll just start with the zip top bags for this year since I don't have a ton of seeds yet. I like the idea of the multi-purpose low tunnel.