Who likes to clean their oven? Not me! So, simply place a layer of aluminium foil all around your roasting dish, “squashing” with your fingers the lower part of the foil to the rim of the dish to “fix” it into position. No need to be precise or artistic. Leave the top open so that your roast can still get brown. Tip: this works better with the thicker/more expensive type of aluminium foil.
This way, the “wall” of foil will contain most splashes.
Another way to prevent the grease that seeps out of your roast from splashing your oven is to place a layer of vegetables at the bottom of the dish (i.e. roughly sliced onions, carrots, etc.) which will soak up some of the liquid and act as barrier.