First, a little background. I bought an aftermarket pad for my 380SE in 2001, from a distributor on EBay. The first picture shows the pad today, with a huge layer of the coating peeled off/disintegrated! Now this is on a car that has been driven less than 1500 miles/year since installed, and only in summer in Canada, not known for blistering heat at any time of year! This is a crappy product, to say the least. I'm sure that current EBay and aftermarket stuff is the same (for example,
http://www.***************/secure/PartImages/1076820326.jpg or the one shown below on EBay. From what I can tell, these are little more than heavy duty pieces of cut carpet underlay. The cost for my 560SL would be over $120.00 delivered, and no different from the junk in my 380SE. F**k that.
Way back in 1999 or thereabouts, I was looking for something to line the hood of my 1940 Pontiac streetrod. I found a product by accident that was described to me as 'cab liner', used to line the inside of trucks and heavy equipment cabs. It's 1/2" thick foam with a layer of black perforated vinyl bonded to it. (second pic below) It's still under the hood of my rod today, in perfect shape. I'm running a hopped-up 350 with headers, so there is lots of engine heat, so this is the stuff to get! It's about $12.00/sq ft here. Go to your local auto upholstery shop and they'll probably have some scrap around. I found it in 2 local ones, no problem.
After doing a search here, I came up with this thread, which I think is an excellent how-to of this job with the hood on the car
Hood Pad Replacement (pics)
I'm going to describe how to cut one from your own pattern, using this super material, and how I did my 560SL hood. The 380SE job was done with the hood on the car, mainly because it has 2 removable metal strips that hold the pad in tight, making the job much easier. But after doing this job, I'd highly recommend removal of the hood instead of all the mess, and messing around and awkwardness with it on the car. Your choice though.....