We recently decided that we weren’t going to spend several hundred dollars on fire bricks for our first rocket mass heater (RMH) in a greenhouse.   At first we contemplated making our own bricks out of perlite  and fire clay (Lincoln 60), but then I posted at the Permies.com forum and I saw Matt Walker’s video:

I hadn’t realized how easy it was to cast your own core and it’s way cheaper than buying fire bricks. We’ll use perlite, fire clay and fire cement, possibly also fiberglass.  The only problem was that we we’re building a 6″ RMH and Matt’s dimensions were for 8″.  There are tons of comments with Matt’s video and several people inquiring about the 6″ dimensions, but I have to admit that I just couldn’t focus as I’m incredibly busy right now. And with our super extra slow internet connection it’s really hard to go back and review videos.
We have  Erica’s 6″ Annex plan, but it shows only how to lay out bricks (which we’re not using) and it doesn’t contain some of the dimensions.
I HIGHLY recommend the DVD Rocket Mass Heaters with Ernie and Erica (this is an affiliate link and we get $15 if you order through this link, but that’s NOT why I recommend it.)   We also have Paul Wheaton’s streaming 4 DVDs on RMH, but I’ve come to hate Vimeo as we have not been able to watch it all the way through.   If you’re serious about building a RMH, it’s worth the extra dollars to get the DVDs.  Rocket Mass Heaters with Ernie and Erica is very well organized with chapters and sub chapters so you can review materials, building — whatever you need to review. It’s there when you need it!

Give your friends the promo code burn to give them $10 off.

That makes it only $37 including US shipping, but I don’t know when this offer expires.
Back to the 6″ core dimensions. 
Our WWOOFer Geoffrey is really good with Autocad and Sketchup and after getting some more info from Matt, he came up with these sketches:

rocket mass inside

The inside form (we’ll probably use 1/4″ backerboard)

Quotation above

We’ll probably use 1/2″ or so OSB for the outside form.

rocket mass box

The outside box

rocket mass total
If you don’t understand the drawings, please watch Matt’s video!  And don’t forget to send him a few bucks for his work.  And of course please post here if you still have questions.
I’m going to ask Matt and the Permies people to have a look at these drawings and to let us know if we got this right.  If so, I’ll post the Sketchup file and you can play with it too.
12/10/14:  Matt had some excellent suggestions and I uploaded new pictures.  And I’ll be looking for different cement, have to watch Matt’s video again.