I am not sure about a do it yourself solution. But sporty's offers several different keyboards that are made for ipads. I personally just let mine lay in my lap and don't have a special kneeboard for it, but they are definitely out there.
http://www.sportys.com/pilotshop/category/1100
Here is their section with the different kneeboards for mobile devices.
Easiest DIY option is a standard silicone backing with holes cut in the sides to slide your old kneeboard's strap through. With those silicone backs running between $4 and $14 on ebay, you won't find much cheaper.
Marshalls and TJ Max definitely have them.
The reason I know about it is that, when I got my iPad, I had an iPro Aviator (http://www.forpilotsonly.com/iPro-Aviator.htm) on order and wanted something for the interim. Except that I liked the iPro's half-height clipboard, I probably would have stuck with my makeshift one for longer (as it turns out, I'm mostly using the Apple SmartCover most of the time with nothing else. At least for now. I can't seem to make up my mind on this one, other than I really don't want a mount)