It is well-known that $u(x,y,t)=(4\pi t)^{-n/2}(e^{-|x-y|/4t}+e^{-|x-y'|/4t})$, $x,y\in \mathbb{R}^n_+=\{x\in \mathbb{R}^n|x_n\geq 0\}$, $y'=(y_1,\dots,y_{n-1},-y_n)$, is Neumann heat kernel of $\partial_tu-\Delta_yu=0$ in $\mathbb{R}^n_+$, $\partial_{y_n}u=0$ on $\partial\mathbb{R}^n_+$.
Is there any other explicit example of Neumann heat kernels?