Looks very cool, watching this unfold.
I can offer some insight to tooling costs. I'm a manufacturing engineer with experience in low and high volume plastics and some metals. You may already know this stuff, but figured it might be helpful.
The 3D printed materials are an option, but you'll need an industrial printer to make anything as strong or heat resistant as I'm thinking you need... The MarkForged printers offer high temp options, but entry price for those is $10k and up to get into the high temp resins. Price per print would probably be really low though. Stuff I've had quoted on their printers is probably around $10 for something the size of your piece - and that still has a markup from the printer service. Doing it yourself would probably be a couple bucks.
Stamping could be great, but you'll still have finishing steps which I assume would add cost making each part more expensive than it needs to be. Throw in the initial tooling cost and extremely limited application AND cost of tooling rework should you decide you need a revision in a bend or something and I'd be cautious of stamping.
Injection molding has a HUGE tooling cost. $5k would be suspiciously cheap for anything that's going to run a dozen or more parts even with a basic A-B mold. I'd expect closer to $15k. This has similar problems as Stamping where revisions will be very costly. You'd be fine with temp though - plenty of molded plastics could hold up.
CNC seems to make the most sense. Your square holes are doable via holed corners or a small bit to finish the corners with a small radius. If you can order 100 parts at a time or more you'll get pricing down a ton. I've used RapidCut many times with great results. The cutting is typically done in China with a level of quality and low price you just can't touch with domestic shops. The customer support is all in the USA so you can't really tell any of it is done overseas until the shipping box arrives with Chinese postage.
Manufacturing details aside, I'd love to see a plug and play drop in kit. Something I could order from you with no permanent modifications required, no core, no sacrificed parts that I could drop into my car in an afternoon. I've been toying with the idea myself for a long time, just never had the time to dedicate to getting it all set up.