In theory a large multi-layer pad with a rectangular hole through it should end up with through plating. Try this and check using gerbv: 1. Select your Board Outline shape then right click and select Convert to Pad;  2. If necessary to meet the JLCPCB minimum pad annulus dimensions, increase the track width (you may have to  edit the shape to maintain the inner hole dimensions);  3. Change the Layer attribute to Multi-layer.

@andyfierman hi thanks for the reply. i have just tried this and it seems that the shape i created does not itself become the cutout but instead becomes the annular 'ring', the cutout is dictated by the limited option of either hole or slot. is this the case or am i missing something here? just to reiterate i wanted to create custom inner cutouts which are plated in the same way as THT pads. (this is purely an experiment for aesthetic value). thanks.

Does your conversion to "pad" offer you the option to represent it as a polygon?

@andyfierman once i have converted the shape to a pad, there is: pad properties > shape > polygon. but the hole shape is only round or slot.

Sorry, out of ideas except have a search on Google for: edge plating easyeda There are a few posts on the subject that might help.

cool thanks for the help