I thought the suggestion was to either use a mosfet or a transistor depending on whether you are actually going to have a PWM on the actual element.
the link you posted is the first part of the request.
Just thinking about it now, i realize that because of the speed at which the element is changing the temperature of it's surrounding, is so low, that you could even do it with just the relays (though they'd be clicking a bit and may not have a huge lifespan, still probably long enough, if you think about windscreen wiper interval relays, they seem to last forever as well)
PWM with a frequency of 1Hz is already way faster than required, i would probably start out with .05Hz and see if that covers it. You may define the resolution to a Minimum of 1 second steps.
This is of course also possible with just a single extra mosfet, though then i'd probably would go for 1Hz
Really all there is to it is how long you turn it 'on' for compared to the time you turn it 'off' for.
Efficiency has been out the window from the start since it's about a Peltier element.