Shopping at not-our-usual Menard's today I chanced into a couple of new (to me, at least) knives specifically designed for cutting foam with no dust or fumes. Second picture is the original find from a couple of years ago.
I think the "Tool Shop" hand knife in the first picture is a Menard's brand. It was on a hanger on the end cap of the aisle with XPS foam insulation, for $4.99. The Bosch sabre saw blades were at the bottom right of the Bosch blade display in the recip saw blade section. The Festool blades in the original discovery were stocked at few U.S. D-I-Y brick-and-mortar stores, so now that Bosch has an equivalent I think they might be a little easier to find.
Here are all three for comparison:
The hand knife is a blessing and curse. The bread-knife (wavy) side can only be used at a moderately acute angle, and therefore for straight cuts only. Cut vertically or anywhere near vertical, and you'll shred everything. For thick foam, 1-1/2" or 2", it works only when there is some "relief"- that is, when one side of the cut bends enough to push away, otherwise it will bind after only 2-3" of cut. The straight side was great for carving and shaping, within limits of the large blade. You guys using disposable utility knives for carving will probably want to keep on keepin' on, although the big knife will be dandy for initial shaping on mountainsides.
One thing that jumped out at me... almost literally... was pulling this knife out of a binding cut. Get in too deep, and the effort required to extract the blade is very hard to control. That's fine, normally,
but this is a double-edged blade! 
Pull out with all your strength, and you have a near-razor-sharp edge headed in your direction, very fast. Conclusion? Good for carving, not-so-good for basic straight cuts in thick foam. 3/4" or less you should be OK for long cuts.
You can sort of figure that you're not getting a lot of quality for a $4.99 knife. The grind on the cutting edges was ragged and not all that precise. It is quite sharp, however, so I'll keep it around for carving projects.
In testing the two sabre saw blades, I really didn't notice much difference in performance. The Bosch blade has a reverse cant, where the bottom of the blade engages the material first. I can accomplish the same thing with the Festool's straight blade using the cant adjustment on my sabre saw. Both blades' cuts were very smooth, and I have no feeling one way or another about the Bosch blade being an inch shorter.
One additional note... I had posted in Gary's
Tehachapi, BC thread about putting a handle on a sabre saw blade. Yeah, that will work more or less, but in tests this evening I discovered it's crap for carving - shred city. Stick with straight blades for the detail work.
