Maybe get a flat sheet in the same fabric from the same line and sew a 3-5" extension by doubling the fabric and making a nice welt or finished end. Or just getting a extra large coverlet so it will drape completely over the bed and put the duvet over it.

Coverlet like this, then put duvet over-
http://www.flickr.com/photos/44713708@N00/4185618931/

http://www.flickr.com/photos/44713708@N00/4177210998/

I was going to suggest a California King size instead, but looking at the website, it is the same size as the King size duvet. I alos realized the photo above is from the website, not your own.

So, just guessing, but is the bed skirt different from the duvet? What if you had a bedskirt that was a really close match in color and texture to the duvet?

I'm not trying to be snarky, but I just wanted to provide a friendly reminder: Odds are good no one else will ever notice this. If it really bothers you (and I can totally understand if it does) sew something on, but I'd say just get a white bedskirt and call it a day.

A duvet is SUPPOSED to show off the bedskirt - it's not a bedspread...

My king-sized bed is on risers also, so I know what you mean: the ratio of duvet drop to bedskirt length, which is usually about 50/50 on regular beds, is more like 35/65 when your bed is on risers.

This is an easy fix. You just need something tall to balance the riser-assisted height of the bedskirt.

If you put something high on the wall above the head of the bed -- a tall headboard, a mounted quilt, a framed four-poster -- you will resolve the proportion issue. If you sew, a simple canopy the width of the bed and jutting out about a foot from the wall at the ceiling (done in the same fabric as the bedskirt) can pull the whole thing together with a minimum of expense.

I think what would be easiest is to make the bed with a blanket/coverlet as the bottom layer. You can either tuck it in or leave it out. Then fold the duvet into 3rds and place it at the end of the bed.

I'd forget trying to alter it and send it back. Then purchase two flat sheets in the same fabric and make a duvet cover of your own by sewing them together. That's assuming the flat sheets are larger than the duvet...

I was going to say the same thing as queenbee1230! This is what I have on my bed and it's lovely. And it's great for warmer nights when the duvet is too much but just the sheet is too little. And on the really cold nights having the extra layer is awesome.