I have purchased several Steel Rule dies over the last 20+ years. I have some material to make up an emergency die if such a case came along. So far it hasn't, but I don't think it would be to hard to cobble a usable die together with fair results on foams etc.
Most small shops tend to lay out the pattern on a chunk of marine grade 12ply. Then use a drill bit in strategic points along a run every 3-5" and at radius tangent points. The holes allow a blade to pass through to start cutting a slot with an over arm jig saw, leaving periodic tabs where the Steel rule material is notched out to jump over the tab. The tabs in the plywood keep the areas inboard and outboard of the rule sections in one piece. For better tolerance than eyeball cut slots in the ply, some shops will have the slots in the ply laser cut. I have been able to get accuracies of better than .010 with the laser cut ply compared to their standard .030 by hand.
The Steel Rule material is pre-bent to the shape of the slot cut in the ply with various die rule bending methods. Then pressed into the slot so it bottoms out at the back side of the ply. Hence the notches in the steel rule will be as deep as the ply. The rule height above the ply at least the thickness of the material being die cut. A backer of aluminum plate gives the Steel Rule some support during a cut. Some dense sponge foam can be placed inboard and outboard of the Steel Rule to act as a stripper of the material being cut.
I have never seen a detailed book showing the process, nor a web site that has much direction.
You could find a Die maker in your area and ask for a basic lesson for simple forms. Most of the dies I have needed were in the $30-$100 range. Not too bad and hardly worth my time to learn how as often as I really need one.