Improve an image mask to be solid with OpenCV

Question

I'm using OpenCV to merge multiple frames of a single video into a single image based on a movement mask. The video is taken by a moving phone with slight hand movement. I was able to align the frames (using feature matching), calculate the background (median), and estimate the movement mask (using BackgroundSubtractorMOG2) but the mask doesn’t give me the perfect movement body shape, and instead, it has “holes”. I’m using that mask to copy pixels from the source frame to the calculated background and not happy with the result because the image has the same whole as the mask. It's fine if the saturated mask is not precise, because all the frames are aligned, I don't mind take a little bit of extra of the source image with the saturated mask.

Is there a good way to do the following mask improvements using OpenCV?

UPDATE: Trying to apply dilatation and noise reduction has the following result. It's not perfect but acceptable. With better noise control/reduction I feel it's possible to fill the largest contour although I still have some empty areas.

And some other example, when the whole object is in the scene, I really want no holes in a person