The best place to create an artificial planet is in orbit around Earth. If you make it in geostationary orbit, then Earth and its double will be tidally locked together, yet still have a 24 hour day each. And because they are tidally locked, they won't be disrupted by each other's gravity.

And that also deals with the problem that the planets are already about as tightly placed together as they can be. Each planet can pretty much clear its orbit of other objects including other planets. So if you added another planet as an Earth NEO somewhere  between Earth and Mars or between Earth and Venus it would be like any other NEO, would hit one or other of the planets, the sun, or Jupiter or be ejected from the solar system within 20 million years.

But a planet in orbit around Earth is tightly bound and won't be lost in that way.

However if your reason for making a new planet is for living area - this is a very inefficient way of getting new space to live. After all most of the bulk of a planet is molten rock, and so not very habitable. Only its thin skin is habitable.

It makes much more sense to just build the skin of the planet and leave out the rest. Of course a hollow planet won't work very well.

But instead build lots of smaller Stanford Torus or O'Neil type habitats.

There is enough material in just the asteroid belt to build Stanford Torus habitats with surface area equal to a thousand planets the size of Earth. Though the mass is only 4% that of the Moon. Our Moon has enough mass for habitats equal to 25,000 planets in surface area. Most of that mass is used for radiation shielding.

Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths