Just an FYI for those who've never held one of those pads and might be missing them in their build, they are made by pouring an expanding (artificial) rubber foam into a mold. The foam then expands through a chemical reaction and fills the mold. They don't look as though they've been trimmed of any excess out of the mold along the dash side, so I would guess that it is a closed mold that ejects excess out of a port somewhere, which is then trimmed off after curing, or they knew how much stuff to pour in to hit the top of the mold. If you look closely at them, they're not straight towards the ends - they have a gentle curve that is molded in, but they would take the shape of the dash with the right positioning of fasteners.
Some version of "Foam-iT" would work, but I don't have enough experience with it to recommend one AND the stuff can be expensive (like $30 - $50 to make a set).
Lacking a mold to work with, everyone we've heard of on here has either made their bumpers out of wood or some other material that will hold the shape and is able to take the shape of the curved dash and which is also able to be stapled to from the rear.
You could also look at Polyethylene foam sheets, say, 1-1/2"- 2" thick and 54" X 24" and cut what you want out of that. The stuff is easily cut with an electric carving knife and it can be shaped either with wood working tools or with a wire knife ( a short length of wire connected to a power source (12 volt battery). When it heats up it slices through the foam like Buttah. You can make one using a decent soldering gun, or buy one of these: