Detergent bottles are generally HDPE (High Density Polyethylene). HDPE is stable at up to 250F. You'll need much higher temperatures than boiling water (216F) to thermoform it.
You may have better luck forming white polystyrene sheets from the hobby shop over a premade form. You mount it in a cheap wooden frame, heat it in an oven set on WARM, and after a couple minutes, with gloves, push the frame over the waxed form. Since (IIRC) the pieces of a portal gun are simple planar shapes, you should be able to get away with doing this without a complicated vacuform machine.
That's the oversimplified procedure, do some searches on thermoform/vacuform tutorials to get more details.