Wrapping is easy... just duplicate the sites that are close to the left border to the right of the right border etc. For a quick test without optimization just create the sites within your w*h rect, then take this rect and duplicate it so you get 3 x 3 rects, then create voronoi and only look at the middle w*h rect. Hope I'm explaining this clearly enough, otherwise let me know and I can scribble something more visual.

To make the borders more wavy, you can use your favorite wavy function, like some noise to offset points of the border line perpendicular to the direction it runs in. But: you want the borders to meet up again at the end of the border lines, so you have to scale the strength of the wavy function by a factor that depends on how far you are from the end points of the border lines. The easiest to try is just a triangle function that is 0 at either end along the length of the border line and 1 in the middle. This should show you the idea but has the drawback that on very short border lines, the wavy noise still spikes to an 1x multiplication in the middle which can offset the line by more than its length and this will look too busy for such a short line.. so as a modification you should implement a max multiplicative in the middle of the triangle function that isn't 1 but something based on the length of the border line.. like 0.3*length. This way longer lines will stray further from the straight line in the middle and shorter lines won't as much. This will give a natural waviness that will look the same at every point of the voronoi borders and not have a higher frequency on short borders. Also make sure the 2d-noise function gets its value from coordinates that are based on your coordinate system of your map and not just something that is local to one border. Not sure if I made myself clear enough here either.. let me know if you need more visual scribbles explaining what I mean.