How to create a custom KEEPOUT area?

[dostr63]

I want to create a KEEPOUT area on a PCB - where there will be no SOLDER-MASK or COPPER, just bare PCB - and for it to have a custom shape.
The only place I can find to set a KEEPOUT area - is in the Pattern Editor - but only for drilled holes and fiducials, and those are strictly circular.

Is there any way of either creating a Pattern Editor part that defines a custom KEEPOUT area, or of creating it in PCB Layout please?
Or perhaps some other way to ensure that there is no SOLDER-MASK or COPPER in a custom-defined area?

(If a custom shape can't be done - then I guess I'm going to have to try to do it with overlapping fiducials, with the copper centres set as small as possible)

Purpose: On a single-sided aluminium PCB, I need to be able to define a line across the board where the aluminium can be bent/folded - so that along that line the board is *only* aluminium, plus the dielectric layer - which AIUI is usually a mineral-loaded epoxy. Obviously there is no tracking near the intended fold-line, it just needs to have a reasonable cosmetic appearance after being bent - hence wanting only the aluminium and dielectric layers in this area.

Note - this is not the same as a route keepout area.

[dostr63]

To answer my own question (because it's taken a while to dawn on me) - create a Pattern editor part that defines the shape needed in TOP MASK.

This actually defines a *negative* area where TOP MASK will be excluded.

Verified working by checking the Gerbers in Gerbv.