Well got some good progress with my underbody plates. I want to split this up into 2 parts. These are made of coroplast which is cheap, lightweight, readily available. Durability and heat resistance will be issues but hey this is a fun Coronavirus lockdown project.
PART 1 - is the plate connected the front of the diffuser which extends past the gas tank and uses the 4 gas tank strap bolts for extra support.
Here is my template. The center is 2 pieces riveted together for strength
Doesn't come out too heavy at 4.5 lbs.
Here are the brackets I made with aluminum heating and bending them on a vice.
Here is how the brackets are used with a black push clip in the bottom
Here is the initial connection to the Velox diffuser. You can see there is a hole from their setup
Riveted in a hole cover
Added aluminum rivnuts to holes that were already on the car
Bolted down with fender washers
So the main issue is that the center will eventually melt. When that happens I will add extra support around the hole.

Pic of some the connections points
Total weight added 4.75lbs
PART 2 making a whole center body plate.
This was easier as it only needed to be a large square. And I only used one layer since there was plenty of connection points underneath
Weight - comes in at 2.5 lbs, Ill say 2.75 with the added fender washers. But also lost 1.5lbs removing the stock plate from the factory. So a net gain in the middle of only 1.25lbs
Connected and overlapping the rear piece
Tried to get a pic from the rear to the front
Pic from front to rear. Will still be connected this middle plate all the way up to the front bumper so haven't used all front connection points
PART 3 will continue on Friday since it will be raining tomorrow.
I already cut out the center of the factory aluminum skid plate and replaced with coroplast. This is not only lighter but gives me easier connection points to the front plate I will be making and also flatter.
Stock Plate weight 4.75lb
New weight 3.25lbs

Underside black was some spraypaint test