QRD diffusor

I have been thinking about making some QRD diffusors, but I have no experience with them. Only know what I've read on the internet. I'm hoping that you guys have some hands on experience that you would share.
I found this QRD calculator:
http://www.digitalaudiorock.com/cgi-bin/qrd.cgi
Does anyone have any ideas what would be best parameters.
Also what materials would be best?
Thanks

I've followed the advice of this site for mine.

http://www.mh-audio.nl/user/acoustic%20calculator.asp

and I've used mdf wood. It worked well for me

Mine looks a bit like this one:

Probably shouldn't post this, but I paid for it so fuck 'em.

Thanks Miko, Mushy and NH.
Check this link out:
http://gendaihaiku.com/studio/N23-test/index.html

if you're looking at getting into building your own treatments / spaces, you may want to pick up a copy of The Master Handbook of Acoustics by F. Alton Everest. Its pretty useful; contains explanations, calculations and some construction ideas for QRDs and just about anything else you may want to treat your studio with, as well as plenty of theory about the why and how of acoustic treatment, and is about 25 bucks new from amazon right now. some of it can be tough to get a handle on, and some of it is a bit cursory, but I find its a good resource. 

The Jeff Cooper book is really good too. It's more of a reference book, but gives a lot of construction details for target specs.

http://www.amazon.com/Building-Recording-Studio-Jeff-Cooper/dp/0916899004

hey Pd, did you make those yourself? really nice stuff!