Blow mold decorations can be quite large in size and therefore prone to catch a lot of wind pressure and if not properly secured could be easily knocked over and blown across the ground causing damage to the paint, shape, and overall quality of the figure. Furthermore, over the last several years blow molds have become scarce and therefore more valuable as some producers have went out of business, which can make them an appealing target for thieves. For these reasons you should make sure when on display, that your blow molds are secured and anchored in place and you can do so by using one or more of the following methods.
Most blow mold decorations have large access holes at the their bases through which you could place a few bricks or sealed plastic bags full of gravel rocks or sand in and around these bases to anchor down the figures when they are set in place. Its important to distribute the weight evenly around the base for proper securing to resist wind from each direction. Remember taller blow molds need more weight in the base for proper anchorage. If the figures are weighted down enough, even on very windy days the figures will remain in place and not move.
Another option would be tying string or twine around the circumference of the blow mold to a large wooden stake or metal dowel driven into the ground on the back side of the figure. This will secure the figure to the stake resisting wind and making if difficult for a would be thief to remove the decoration.
Alternatively, the stake could also be staked into the ground to position in the center of mold through its open base. After the blow mold is set in place, when the wind blows, the stake (if tall enough) at the interior will prevent the mold from toppling over due to the wind. To protect the mold the stake should have a cloth wrapped around the top of it to prevent scratching or denting the interior of the figure as the wind blows. While this technique works well against wind it provides little resistance to theft.
Using a thin metal chain lock running through the light socket opening at the back of the blow mold to a plastic or metal stake driven into the ground behind the mold combined with a based weighted down with bags of sand or gravel is another good method that will provide you with ideal anchorage against both wind and theft.
You could also try using twine or string tied around the top of the figure on all sides and staked down in all directions in such a manner to keep the figure from moving. This is somewhat like how a guyed antenna tower is supported and provides great resistance to lateral movement from wind.
These are just a few ideas to help you properly secure and anchor your lighted plastic blow molds for your outdoor Christmas light display.