I bought a kit of the black gummy stuff that was pre-cut and installed following their map, the different shapes and sizes didn’t completely cover the panel, just the right amount to deaden the panels. That was 30 years ago and I don’t recall the company, I probably purchased from one of the model A suppliers.
This is a common misunderstanding of how to use the sound deadening materials.
You do not need to cover 100% of the metal surface to get a good improvement. Anything around 75% coverage gets you the benefit. Above that you reach the point of diminishing returns.
So you can save on materials, but even more on labor, by just cutting pieces that cover the larger areas and stop there.
The dense tar/metal mat is designed to reduce "drumming" of the metal. Think of a large sheet of metal that you hold up by the corners and vibrate to make a "thunderstorm" sound. That drumming is what you are reducing by adding this sticky metal mass. The full sheet does not need to be covered to stop the drumming.
But to reduce sound and heat in your car, you can't stop with the sticky metal stuff, because that only reduces drumming. You have to put a layer of sound and thermal insulation on top of that.
The people that make DynaMat (the sticky stuff with a metal layer) also sell a self-adhesive insulation called DynaLiner to go on top of the Mat. The Liner comes in 3 thicknesses, up to 1/2" thick. This stuff, on top of the Mat, really deadens sound and reduces heat transfer. It sticks to the Mat and stays there. Here you DO want to shoot for close to 100% coverage.
There are probably other suppliers of equivalent stuff. Use the thickest Liner that will fit in the space you have on top of the Mat (behind interior trim panels, etc., out of sight). The two combined really kick it.
Under my front and rear floor mats, I use a non-adhesive sheet called DynaPad. It is only 3/8" thick and it is super sound and heat deadening. No sticky metal layer, no self adhesive anything. The DynaPad is dense and lays down flat under the car's front and rear mat or carpet. Easy to remove to work on the car. No one knows it's there, but it is super effective at reducing noise and heat.
It is not cheap, but you get what you pay for.
