Cheap homebrew lab PS; need verification of magnetics for toroid
Your secondary is 14 ga, which has a cross-sectional area of \$2\ \mathrm{mm}^2\$.  You spec 16 turns, for a total secondary area of \$32\ \mathrm{mm^2}\$.  The area in the center of the toroid is \$\pi({2.91\over{2}})^2\ \mathrm{mm^2}\$, or \$6.7\ \mathrm{mm^2}\$.

That's going to be a tight squeeze!