I know this isn't quite the right place for this, but I also didn't think a new thread was warranted. ...And I'm not starting my 'Build Journal' until I have a design set.
Anyway...
I tested a minor assembly today. I did not like what I saw.
Afterward, I did a high-dive into the depths of the interweb (in the few additional moments when I had the time), which prompted me to do some examination and testing.
The "Poplar" that I've been buying is actually Cottonwood (one piece) and Quaking Aspen (the rest of it)!
Too soft. Too rot-prone. Too wrong for longevity.
I do not see myself using "Poplar" in this trailer, now, unless I can trust the source (which I can't, yet). I'd rather use pine. I have a lot of experience with the above wood families and I no longer trust "poplar" for this application.
I think I'll be turning to oak where it matters (attachment/mount points, etc.) and possibly pine where ultimate strength and screw-holding ability aren't terribly important.
We'll see how that plays out over the planning stages...
In other news, I found supplier of 4x8' 1/8" plywood (rather than 5x5). They even let me play with their product. There was the traditional Baltic Birch. But also "Oak". It didn't look like oak, smell like oak, or feel like oak to me, and had a 'Made in China' label on it. I'm not sure that I trust the oak; but at least there's another supplier on the table.
We'll see how things work out.
If I do go with Oak in any fashion, that will make weight an even bigger factor to keep in mind. (Oak's weight penalty, vs "Poplar" is about 50%.)