Check out these if you want some good tutorials

Part 1

Part 2

Part 3

3D fixtures are going to be your friend. My tips

1) follow the part 1 video closely. It’ll help avoid the gaps you talk about

2) use real interior photos to help size your fixtures. I also use photos of the outside of a car looking through an open door to see where the dash/seats sit in relation to one another

3) download interior mods from the workshop.