Placing an oven on plastic grates is not recommended and could be unsafe. Ovens generate a considerable amount of heat, and plastic is not designed to withstand such high temperatures. If you use plastic grates to elevate the oven, there's a high risk that the plastic will melt or deform due to the heat, leading to potential damage to the oven and creating a fire hazard.
When installing an oven, it's essential to follow the manufacturer's guidelines and specifications for proper installation. If the new oven doesn't fit in its designated space, consider alternative options such as adjusting the cabinet or counter space to accommodate the oven properly. If necessary, you may need to consult a professional contractor to help modify the area to ensure a safe and proper fit.
Safety should always be a top priority when dealing with appliances that generate heat, so it's best to avoid any makeshift solutions that may compromise the integrity of the oven or pose a risk to your home and family.