IMO, nothing you did could be considered proprietary. Heck, we are a jury of your peers in the business and nobody has yet to say this is proprietary, and I would not see a mediator/jury seeing it any other way. Legally I know these things are very hard to prove and most attorneys don't touch them unless they are pretty clear cut or involve a lot of money.
Connect's example was one I was thinking of when I first read your post. I think to prove something is patentable or even proprietary there has to a level of uniqueness and sophistication above and beyond what a typical solution may be. Otherwise, Connect would have patented every moment and unique connection he has ever designed. His side plate example shows you what is probably required to get a structural detail patented, it is way above and beyond your standard connection and there was presumably a lot of testing involved as you could not rely on standard calcs. Well even AISC's shear connection design relies on significant testing, but they do not trademark their work thank goodness...
I have designed all kinds of connections that I thought were just awesome, but truth is they have all been used before. Give several structural engineers the same problem and you'll probably get the same 2-3 details. Reinforcing these containers using HSS, plates, extra welds, and developing these details are a normal part of that type of business and probably similar solutions that other engineers are using for other companies. We all have typical details in our libraries for situations we encounter all of the time, and they are all probably pretty similar.
Now you cannot go down the street to company B who is doing the same thing and divulge everything you learned, at least not for a year (non-disclosure AND non-compete violation). In that unique business I don't even see that being an issue, there probably is no company B down the street. I would also think legally any details in CAD, any actual calcs or FEA models are THEIR property, they paid you for that work product and they own it. That does not mean you have to submit to a Men in Black mind eraser device, but you'd have to recreate that physical work (over this coming year, you have the time). I did not forget the details all my previous companies used, I simply recreated them and calc'd them out for my own. This is how structural engineering as a profession works.
NOW- I will dream up an example. This company had you develop some type of connection system that to everyone's knowledge had never been done before, especially not by your competitors. You do calcs, FEA, etc., and then you guys go in the lab and test this design to prove it works. Now you have something proprietary and patentable I would think, and this would be completely off limits. Now if they did not patent it, and one year went by, and you got clearance from an attorney, then....
Like everyone else said, consult an attorney, tread lightly for a year, and use it as pencil sharpening and idea time... I think the non-compete portion of the contract is what keeps you from doing this type of work for a year more than the non-disclosure.