How to mechanically join edges of boards forming octagon?

[Mayo]

I want to join the long sides of rectangular boards in the shape of an octagon. I can cut the edges at 22 1/2 degrees so that they assemble correctly. It's actually an open octagon using only 5 boards instead of 8.

Is there a way to cut the edges so that it's a mechanical connection like a dovetail or maybe like how a glue joint bit cuts the edges?
Maybe biscuits or a spline or a particular way of using a dovetail bit with the boards angled in a way that lets them join correctly.
Or some kind of cad created joint? Maybe that new dovetail joint program with the boards positioned differently? I'm lost...

I can't picture how I would angle the boards in order to create a secure joint. Cutting the 22 1/2 degree cuts on my table saw is not a problem - the way to make a joint that locks together and still achieves that angle is the problem.

Additional info:
The boards measure about 12x25 inches. They're 3/4 inch thick.
And they're not wood - they're 2lb density EPS foam.
The joint doesn't have to look pretty or fit so tight that it's impossible to assemble without breaking the foam.

I want the joint to prevent the boards from slipping towards the inside or outside of the octagon - vertical slipping is not going to be an issue.

Any suggestions would be greatly appreciated!

[srwtlc]

Something like this bit should be just the ticket, unless I'm misunderstanding the application.

http://www.eagleamerica.com/product/192-4505/ea_-_glue_joints

[steve_g]

Mayo...
Are these EPS parts flat or on edge?

SG

[bleeth]

Mayo:
Lay the planks down next to each other with the outside up and tape across them with masking tape. Flip the whole assembly over and put adhesive in each joint and fold them up. As you get to the last bit, put adhesive on the ends, bring them together, and tape them closed. Let your glue set up and then remove the tape. This is a tried and true method of miterfolding any board material. You simply have to use adhesive that bonds well to the EPS. I would imagine that just about any of the epoxy adhesives would work or even silicone adhesive such as 3-m 5200 or 5300, but I am not well versed in adhesives for this kind of foam. Just be sure not to use something like contact cement as that attacks the foam.

[Mayo]

That miter bit could be exactly what I need!

My intention is to make a product for resale which the customer will assemble, so it has to be fairly idiot proof. The lock miter bit could provide that ease of assembly for the parts - provided I can cut the foam without tearing out chunks or melting it.

Although those bits are a little costly, I may get one just to try out this idea and if it doesn't work then I'll have to make some octagon planters or something.

The joint is on the long sides of the rectangles - 25 inches long.

Thank you to everyone with the suggestions - as usual Shopbot people and the forum are the quickest answers!

[Mayo]

Wow - I just found one of those bit sets on Ebay for $37.95 including shipping. This will be a cheap way to make a test run on my foam.