You're getting steamed trying to make an intricate cut with your portable jigsaw. There just doesn't seem to be any way to support the workpiece firmly.
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A piece of 2"-thick Styrofoam insulation board will cool you down. Lay it on the floor or workbench, with your workpiece on top. Your workpiece won't slip around, and it will be supported on both sides of the cutting line. A standard-length jigsaw blade won't cut all the way through the 2" foam board so you can cut even thin materials. The insulation board will last a long time before it needs replacing.
–Pierre Duval, Quebec, Canada