Simple answer: no.
In no particular order:
The Casimir effect is not usually understood as acting "on" boundaries, but as a phenomena that exists because of them.
The Casimir effect is not known to give rise to mass densities of any kind... it is usually thought of as a reduction in the energy density of the vacuum between conducting plates.
The scale that the Casimir effect is significant over is very small (nonexistent on cosmological scales).
The mass density of the Universe is not constant: notice vast regions of almost no mass and very small regions with stars and black holes and stuff - that is just something that is fed into certain models. In this sense the uniformity of the mass/energy density is about the statistics of the clumping ... it is likely an effect of a symmetry principle, we know that rapid expansions have this effect, and we know the Universe is expanding.
There are no known boundaries to the Universe - most recent CMB data suggests the Universe is, overall, infinite and flat.
So basically every single premise in the speculation is false in some important way.
However, if you have more than speculation to back up the premises, please share.