Best way to create a simple and economic steam machine for steaming wood?
The container doesn't have to be a PVC pipe. Wood boxes were the traditional solution, and still seem to be more common among woodworkers than the PVC approach. 

There are steam generators available from woodworking stores which are a bit more effective than an electric kettle. They may or may not be significantly more energy-efficient; boiling water takes the same amount of energy no matter how you do it, so the only question is how much energy gets lost to the environment rather than going into the water and being kept in the steam.