Not a bad idea, but there are alignment lights at the four corners of the mail slot to indicate when you are aligned with the slot for entry
Yea? I have to check. I mean, yea, i am new player who barely spent a few hundred hours in the game, so i guess i am excused for not noticing it yet... now i am curious.
On the suggestion itself: hmm, it doesn't sound too bad, albeit complicated when implemented really well. I mean, the game can treat the ship as one box and display distances to to each side. That's simple enough to implement, but the mail slot for example is not a pure rectangle but actually gets a bit smaller to the sides. Thus the rectangle based display might suddenly indicate negative distance, but the actual ship still fits through.
Alternatively you can work with distances to the ships actual surface. The problem of what to display of that hundreds of data points is still simple: the smallest of them all. But considering how complex some ships geometry is and that you are not merely checking for collision (single point connection) but distances (multi-point connection), this might actually turn out to require much more performance than you'd expect.
So yes, when tinking about how actual distance sensors work: it's amazing how much physics do for you by wave distribution and reflection.
So all in all i see how the distance indicator might really help. Especially newer pilots, when they first time get into a big ship. (I still remember how often i scraped my cutter along the walls of the mail slot, as i missjudged the up and down distance. ) But i guess for any performance efficient implementation we might have to accept some oddities here and there, mostly the sensors sometimes telling us that distance is zero, while you'd on the outside view see that you actually still have a few meters spare.