Can i make a square hole in my floor of my garage for a work hole.

so im not sure if this is the spot to ask this but ive builded a garage and would like to have a hole/trench under my car so it looks like the can work the car from below.
terain editing could be but the cursor is way to big i need a 1x3 hole.
i tried a basement but thats to low. it needs to be like 2 full grids low.

i hope its clear if not please ask me. since if got a lot of haevy argument on other forums because they didnt understand me.

thanks in advance