How to attach round light to rectangle box?

[StaggeringGoat]

I have a place for a light out in front of my shop, but there is no light installed. Right now it's just a rectangular box like this with a dedicated 20A circuit:

I want to buy and install a light like this:

http://www.homedepot.com/h_d1/N-5yc...0053&langId=-1&keyword=100+watt&storeId=10051

As you can see, the light has a circular base. My box is rectangular. What do I need to mount that light on the rectangular box? I would prefer not to change the box to a round one...

 

[abnorm]

Check to see if the lampbase is removable and change it out to a rectangular base........

 

[pattenp]

One of these...

 

[acer66]

One of these...

This is what I used when I had an similar issue with a motion light.

 

[RECox286]

X2 ^^^

Uncle Bob