poly2mask not creating filled mask

I am trying to create a mask from the attached image (sampleEdges.png), which was obtaiend by applying the edge command to the sample image (sampleImage.jpg). I have obtained the coordinates of the area I want to mask using regionprops. The MaskBoundary looks like as shown in MaskBoundary.png. I now want to create a mask from the MaskBoundary coordinates using poly2mask, but it does not create a filled mask (mask.png)

sample = imread('sampleImage.jpg')
sampleEdges = edge(rgb2gray(sample),'canny',0.1,25);

regions = regionprops(sampleEdges, 'all');
[~,index] = sortrows([regions.Area].'); % Sort by descending area 
regions = regions(index(end:-1:1)); 

MaskBoundary = bwtraceboundary(sampleEdges,[regions(2).PixelList(1,2) regions(2).PixelList(1,1)],'n');
MaskX = MaskBoundary(:,2);
MaskY = MaskBoundary(:,1);
Mask = poly2mask(MaskX,MaskY,size(sample,1),size(sample,2))

The two regions are the ones that enclose the red markings in ROI.png. But regionprops can only find the right one.

Answer 1

[Image Analyst]

Use bwboundaries() instead of bwtraceboundary().

[Afzal]

With bwboundaries() I cannot specify the start point, and it does not seem to find the region I want. I am also having issues getting the left ROI (see edited post)

[Image Analyst]

How do you identify the one you want? A week or two ago I gave you code to identify the curved bar. Why did you discard that approach?