I have to do a cross cake next week, and the customer has requested that it be covered in fondant.

I've only covered round cakes before.

Any suggestions?

I've done this before. I rolled out a rectangle piece big enough for the entire cake, and then lay it on. Start with the 90 degree angles first (where the cross intersects) and smooth those in. Then do all the corners of the cross, and then work down the straight sides. It takes some patience, but you can do it. I just can't smooth away the seams when I piece MMF together, so I try to do everything in one piece. HTH!

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As long as you have enough fondant to plat around with,you need to tuck in the corners first before the whole cake,fondant cannot be too thin as well,so that you have some flexibility.Good luck.

I have never tried this method myself but I heard it works great for shaped cakes its using poured fondant..i believe it's just called pour fondant!
HTH