Larger capcitor. I think 880 uf is a standard avialble size, should give roughly 8 Seconds.
But the time constant will be a combination of the resistance of the relay coil (which you can't really change) and the capcitor.
If you use a different brand of relay you may get a different resistance ..but it should not be a huge difference.
You can also increase the capacitor value by putting capcitors in parallel.
Of course, you must keep in mind that capactors of these value are polarized. ..i,e there is a positive and a negative side..and you MUST put them in the correct polarity.
My circuit has now been working flawlessly for over a year.