I have had my avionics guy make a harness based on what I plan to install and interconnect. it can be pretty inexpensive and money well spent.
For DIY panels, the harness work is a good one to farm out.
I used the Approach FastStack hub and cables on my RV-10 and recommend them highly for cost, simplicity and expertise. It's like working with a very experienced avionics shop that specializes in exp panels (though I think the core of their biz is certified stuff). I listed the equipment, asked a bunch of questions that were all answered authoritatively, and they sent my a customized hub and all the required cables along with a full set of high quality documentation.
Five years later I am still thrilled by my DIY panel enabled by FastStack. When I added a new box, I called them up and they provided a a new cable, expertise, and updated doc for the price of the cable.
If you want to buy and install a custom panel, see Stein or your local shop. If you want to DIY, save money, and know the work as well as you know how your wing was built, and yet avoid the integration birds nest, checkout Approach FastStack.
For ref: 3 GRT HX displays, G430w, SL30, PS panel, TruTrak AP, G327 Tpx, NavWorx ADSB. Hey, I did it back in 2009.
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