side plank technique question

[ratcreature]

I've been doing the easier side plank version where you are on your knees and can hold that for a while now and wanted to switch to the more difficult one, where the support is your feet, but I keep slipping or wobbling and it is uncomfortable on my feet to try to support my weight on just the side of one foot. How do people do this?

In most of the pictures I've seen people wear sport shoes, and I'm doing my exercises at home on a mat barefoot. Is that the reason, that the shoes stabilize the feet? Can I position the feet some other way for something more stable that's still harder than from your knees?