mylar over round mandrels works too. The mylar can then be removed from inside the tube. If you're using a motor tube as the mandrel I usually use to wraps of 2 mil mylar. That will allow for some variation in the OD's of various motor tubes.
make sure you only use tape to secure the mylar at the very ends of the mandrel That you can get to after the layup. ANY tape holding the mylar in place under your layup will create a night mare
The PVC tube you see in the video is only supporting the motor tube/mandrel.
Also. PVC is not the truest round stuff there is. I would recommend un taping the mylar and trying to spin the layup and mylar while it's in it's "green" state and sliding it some..not off...before it totally sets up. I would do this a few times before it is fully cured
Tony, where do you get your mylar? Going to be rolling a 38mm tube soon.