How much light?
Ultimately, the answer to your question just depends on how much light you want to put out. Each one of the spotlight LEDs puts out roughly 1200 lumens, so you've got options to put 1, 2, 3, 4 (Combo Light), 6 (Six Pack), or 7 (7 Stars) in there, depending on how much room you have.
There is more than one variety of Duckworks mount... the best fit/least modification for the rectangular hole mount is likely the Double Spot, as seen in ArlingtonRV's excellent writeup. If you've got the Duckworks mount with the round hole, the Combo Light (4 spot LEDs) is probably the easiest.
That being said, there's no reason you couldn't enlarge the hole in the Duckworks bracket or fabricate a plate to reduce the hole to fit the light kit that you choose.
As mentioned elsewhere, we've got these light kits on our store (flyboyaccessories.com) with stock here in the US, or you can get them directly from Flyleds (flyleds.com) in Australia.