[Solved] Making an arch - editing the shape of it's 'hole'?

So I made an arch thanks to studio tutorials and I was happy and proud until I got into the game.
It works perfectly well except for one little detail. When I place it, it shows the 'hole' of a base game arch and I'm confused as to what do I have to edit in order for it to show up correctly. Can someone point me in the right direction?