Custom RGB color space

I am plottiing the measured Lab colors of a ColorChecker SG chart in 3D as you can see below:

To obtain the "gamut surface", I use the following code:

[r,g,b] = meshgrid(linspace(0,1,20)); % linspace(0,1,50)
rgb = [r(:), g(:), b(:)];
lab = rgb2lab(rgb, 'ColorSpace', 'adobe-rgb-1998', 'WhitePoint','d50');
a = lab(:,2);
b = lab(:,3);
L = lab(:,1);
k = boundary(a,b,L);
h1 = trisurf(k,a,b,L,'FaceColor','interp', 'FaceVertexCData',rgb,'EdgeColor','none', 'FaceAlpha', 0.30);

Works great as it allows me to analyze how the ColorChecker SG chart "fit" inside a given RGB color space such as AdobeRGB. But as you can see by the screen capture, some points lie outside the AdobeRGB gamut? I don't want to use ProPhotoRGB as I find it is so "big" (I'm still not sold on it).

My question is: would there be any way for me to specify custom primaries? As in Photoshop?

The goal would be to find which primaries are capable of fully enclosing my ColorChecker SG colors and give me the "best" editing space to transform my RGB images from my Nikon to Photoshop using a custom camera profile.

Thanks in advance for your help.