A simple 8 channel denounce circuit board. Wire your buttons (or relays) to a positive voltage, and bring them (along with a ground reference). The output of the board will be a nice clean transition from high to low (and vice versa)

The filtering circuit is a first order low pass filter that operates on the equation:

$$f_c=\frac{1}{2\pi{RC}}$$

Where $f_c$ is your desired cutoff frequency in Hz. A handy sage mathematics script was written to easily calculate and experiment with values for the 1st order filter: