Ok, here's the 2x 555 timer version, as safe as I can reasonably make it.
R1 provides 12mA through the switch and 12v across it when open, so unless you use a particularly crummy switch you don't need to worry about wetting current.
Fs1 and R3 provide protection against shorts to ground in the switch wiring. If Fs1 trips, IC2 is locked out via its Reset pin. R4, C7 provide a power-up delay to ensure U2 doesn't pulse the relay at switch-on, and in conjunction with R3 also provide ESD protection to its Reset pin.
R2, C1 provide input filtering for debouncing and in conjunction with R1 provide ESD protection to U1, which is wired as a simple inverting Schmidt trigger with 1/3 Vcc hysteresis
*. Its Out pin is the inverted debounced switch state (i.e. low when pressed).
R5, C4, D3, R6 are the negative edge trigger pulse shaping network, that only triggers U2 on a new button press, once the current timing period is complete.
C5, R7 is the timing network and R8 limits the current into the Discharge pin. R9, R10, Q1 is a 'bog-standard' relay drive circuit, though its imprudent to rely on the diode inside the relay coil to protect Q1. If you are feeling moderately paranoid, adding a 24V Zener, cathode positive, from ground to Q1 collector, + a fuse in series with the feed to the relay coil wouldn't be a bad idea.
I've minimized the number of values used for BOM reduction. If that isn't an issue, R8 and R9 could benefit by being reduced, and the CV pins don't need so large decoupling capacitors.
LTspice sim with some nasty simulated contact bounce attached. Zoom in on the edges of V(in) to see it!
For the details of how the SIM only Switch Control works, see
LTspice: Piecewise Linear Functions for Voltage & Current Sources and my comments on simultating switches
[here].
H.T.H.
Ian.
Now everybody's got a chance to bash my ideas to death while I chill and hopefully we all learn something!* A buffer following a RC filter for debouncing *MUST* have hysteresis.
You should probably (re)read Jack Ganssle's excellent article on debouncing: http://www.ganssle.com/debouncing.htm especially part 2 where he covers RC hardware debouncers.