It’s sprinkler pipe from the hardware store. You can heat it up and bend it to shape. You can also use something called “foam backer-rod” from the hardware store. Both are very cheap, but the foam backer-rod is soft already so you can just superglue the two ends together to make a circle in whatever size you need. No heating required.

Thanks for the interest. I'll include some suggestions for simple DIY medaka stuff in my next video.