Simon,
Are you looking to insulate for sound or heat or both? Dynamat does make Dynamat Extreme which has an aluminized layer on it. That is what I used on the outside of my tub where it was vizible. While Dynamat and similar mastic and asphalt based products do provide very good sound insulation, they really don't provide much in terms of heat insulation. Most of the insulation material that I am aware of would not stand up well on the outside other than some of the ridgid foams, which sounds like what you've just taken off.
What you might consider is what I used under my carpet on the inside. I used layer of high temp mastic (yes, in addition to the Dynamat on the outside, but I have this thing about my car not being a noisy "kit car") and a layer of closed cell fire retardant polyethelene foam. It is not too expensive and works better than your typical open cell foam used as carpet padding. There is also the aluminized ticking material that is inexpensive, but very thick. This gives both heat and sound insulation (again inside only use.) There are some aluminized silicone fabric heat shielding materials, but these are very expensive and probably over kill for areas not exposed to engine/header heat.
You might try
McMaster-Carr for some ideas on different materials. They have about 5 pages of different types of insulating materials. I am not sure how cost effective it would be to buy from them since they are located in Atlanta, GA, but you will get a good idea of the materials available and their relative costs.
Good luck!
Lynn
