I built my own waist harness along the lines of the original Ride Engine customs. It's been going strong for 5 years now. It's not at all difficult.
Here's how RE did it:
1) Make a thermoplastic mould of your back
2) use the outside of the mould to form a custom harness
Here's how I did it:
1) Make a thermoplastic mould
2) Use the inside of the mould to make a fibreglass version of my back
3) Use outside of the fibreglass copy of my back to form a custom harness.
I used 5 layers of carbon and 4 of kevlar. I'd do it that way again. It is stiffer than the carbon shell harnesses I have hand flexed, but not enough that I feel like I'd want to reduce the number of layers. The reason I put the kevlar in there is that I liked the idea of having layers of less brittle fabric in there in case of failure. Kevlar is annoying to work with, so you may or may not wish to do that. You should vacuum bag the layup while curing.
For the spreader, I use a Jay bar. It is stainless, so very heavy. But it comes with a very nice pad.
To attach the spreader, I use 2 inch wide webbing. There's a loop of that with something like https://www.securefixdirect.com/25mm-st ... 5337-p.asp on it.
For padding, I have a single layer of 5mm neoprene tacked on. There are definitely better ways to do this. My intent was to give the whole thing a try, cut the shell down to the size I needed, experiment with different padding, etc. 5 years later, I still haven't bothered to change anything as it works so well. This is one thing you might want to improve on, especially if you ride with a lot of power or without a wetsuit.