So I spoke with a local box company and their bike boxes are rated 275#. But they are still single wall.
So I see you can buy sheets of cardboard in different thicknesses /strengths. Maybe buying and fabricating a "custom" box???
Thoughts?

Maybe do a little dumpster diving ahead of the airport? Nearly everything delivered to retail stores these days is in cardboard boxes. In the alleys behind these stores, there will be plenty of dumpsters with all sizes of boxes discarded for recycling. Just buy a roll of packing tape and box cutter for a few $ and fashion your own multi-layered box. I've built a few custom boxes for bulky stuff sold online - it's pretty easy.

As an ex bicycle store owner for over 12 years, I can tell you that most stores recycle/ throw away hundreds, if not thousands of bike boxes a year that are made just for boxing bikes. Call around -- usually free.

I work PT in a bike shop and have access to lots of boxes. What I am contacting is a heavy cardboard than the typical box we get new bikes in.
My thought is to pay a bit more for a more substantial material and take time to make a better box.

How heavy are these "Heavy duty boxes" ?

Bear

You could always start with a bike box (or TV box), and stuff extra sheets of cardboard into the sides of the box.

I suppose there is some risk of penetration, but I think the biggest risk to bikes is stuff moving around inside the box, and/or things like derailleurs inadequately protected.

If I had my way, I'd ship the frame and wheels in separate boxes. Perhaps that would even work on an airline if you have two "bags". Just stuff them with laundry and other stuff too.

If you really want a tough box, then hunt down a plastic bike box, either new, or used on Craigslist. That is, assuming, you have a place to store it at the destination.

there is nothing wrong with a regular bike box. In over 12 years, I never had to make a claim about a damaged bike that came in a box, and they went through either fed ex or ups. Not even counting the years that I worked at other stores.

so just stuff a liner box inside a slightly larger outer box. finished.

I got a bike box from my LBS. It was one of the larger cardboard boxes, I think they said an e-bike had been shipped inside it. It weighed 14 pounds. My bike, with frame bag and ExtraWheel trailer all fit just fine, weighed 49 pounds. I wrapped a couple turns of paracord around each end, for ease of handling. Tie knots that can be easily untied, and don't go overboard using duck or packing tape. Airport security will open the box to see what's inside.

only in the land of the free.

here in communist states and military dictatorships,
they xray your bike box and send it on.
no tsa thugs stealing stuff and fondling your junk.

What about all the Plastique .... Ummm Carbon Fiber parts on the bike?

I did not know that. Maybe they do that here, too.

So if I have a choice, is it better to have my junk fondled or x-rayed?