I made several of these this summer for the same reason you want to make them. Follow the link below for another link of some pictures I took while doing it. One tip I can give you is to remove the bottom of the oatmeal carton or the weight of the concrete will cause the bottom to sag. I made "hollow" pilars because I wanted drainage for my planters. I used storage containers for a couple of the bases and jerry-rigged another from a plant saucer and some plastic edging I found. I wanted to give the pillars an ancient look so threw occasional handfuls of chicken grit in the form as I was filling the oatmeal containers. Hope this helps.

Here is a link that might be useful: My columns and planters

Fleur, your pix are absolutely awesome. Admire the green trees in the background. I live in Southern CA where it is dry and very little green unfortunately.

How do I make the pillars hollow inside? If I were to put an empty paper towel holder inside, would that work for making the center of the pillar hollow?

Re hollow pillar...I formed tubes out of fine hardware cloth and made it about 2" smaller in diameter than the oatmeal container. That gave me a pillar about 1" thick. The hollow pilar could then also be plumbed to make a fountain. I think the paper towel tube would work too but make a much heavier pillar.

To also make the base hollow, place styrofoam round on the bottom of your food container, line the base container with plastic wrap (over the foam), put a plastic wrap cup in the center, fill food container with cement mix. The plastic wrap makes it easier to unmold.

I'm thinking the paper towel tube will mash together as soon as the cement hits it. How about PVC pipe inserted for drainage. Several 1 inch or maybe one larger piece. I have used 3 litre coke bottles, top portion removed and taped together with re-bar inserted for strength for the pillars. For the base I wrapped the bottom of the dried pillar with plastic and sat it in the center of a large flower pot saucer filled with cement. Worked great for my birdbath. As soon as the cement sets up remove the pillar. When dry, you may have to sand a little. It is less fragile when made in two pieces.