Tinkercad: How to Fill a Hole 

Thank you, this is exactly what I needed. Many thanks for taking the time to make this and making it so simple.

I don't know why your design is showing like this as it's not actually what happens. it still shows the outline of the piece you placed as the hole when you make it larger despite being perfectly flush. i'm curious as to how you got it to show as one solid piece though

This simply doesn't work like you're showing here when I do it. I'm left with the edges of the new shape after I group the objects. And there is no flashing at all, and it seems impossible to match the surrounding surface.

Worked perfectly fine for me