The stooge holds your plane while the rubber motor is wound. There are several you can buy. Many P30 flyers build their own.
Starlink-Flitetech Sturdy Stooge- available here
Retro RC Universal Winding Stooge- available here
A stooge that could be modified to handle P30's. It would work great mounted on a tripod.
The Morrill Simple Stooge is popular and used to be readily available. It's currently out of stock at FAI Model Supply (9/20). You might be able to find one for sale if you keep your eyes open on the various Free Flight forums and FB groups.
Dylan Cliff Roberts 3D Printed Stooge. More details and downloadable files can be found here.