Nah, dont use board outline, this could mess things up.
Just paint a solid region and then change its type from "solid" to "board cutout". Its that simple, and it works flawless with jlcpcb.

Using the Board Outline tool is perfectly acceptable too.

Note however, that rectangular holes will have radiused corners:

https://jlcpcb.com/capabilities/Capabilities > Drill/Hole Size > Rectangle Hole/Slot

I do not know what the minimum corner radius is.

Thank you for both your comments. The corner radius is fine, it is even a very nice feature I think. Thanks again. I will try it on my next pcb run :)

@andyfierman @kristian.borgstedt Hello
About the customized shape drilling, you can draw the shapes using the (board outline) and we drill it as long as it is inside your main board outline (you can check this using the 2D viewer before placing the order)
About the radius, for holes and slots which has a larger size than 1.6mm we use 1.6mm drill tool to mill it and the radius corner for such shape will be 1.6*0.2=0.32mm
For the lower sizes the corner radius will change regarding the shape size.

Rectangle Hole/Slot	Don’t support	We don't make rectangle hole/slot,the rectangle hole/slot will be made as round or oval hole/slot by defalut.

@JLCPCBsupport,

"...for holes and slots which has a larger size than 1.6mm we use 1.6mm drill tool to mill it and the radius corner for such shape will be 1.6*0.2=0.32mm"

How can you mill a corner radius of less than 1.6*0.5=0.8mm with a 1.6mm diameter drill?

"For the lower sizes the corner radius will change regarding the shape size."

Can you specify how the corner radius changes with hole dimensions below 1.6mm?