Pasteurizers for steaming bulk amounts of coco medium/substrate don't have to be all that fancy or expensive, like the more complex ones with heating elements and PID controllers, etc.
You can DIY a small scale version with parts you might already have laying around.
Simply hook up a tube from a pressure cooker directly into a plastic storage bin (large insulated cooler would be even better!), and toss the coco right into the fabric grow pots. Then set them down in the bin on an elevated rack, and let it rip. Monitor temps with a large meat thermometer. EZ!
Can also use an electric wallpaper steamer unit along with a digital temp controller, or an automated instant pot to generate your steam.