According to PipeDown :
Since you are painting it, you don't need to be picky about appearance.
Almost any wood that had been Kiln Dried should be OK to use. Maybe you can
find something in the Molding isle at the hardware store or lumber yard.
You could also rip the pieces off of some Shelfing stock.

Poplar would probably be a good choice but most woods are stable once
completely dried, especially once sealed with paint. Red Oak would also
work well, as it is harder than poplar and may be stronger considering 1x1
construction (which I question but will ignore that since I can't look at
your actual idea). Less likly to split from the fastners or crack along the
grain.

Poplar's advantage is not only is it very stable under strongly
varying conditions (it's why it's the preferred material for wooden
screen doors), it's _real_ cheap to get in large clear pieces, easy
to machine, stain or paint.

Even KD'd (allowing for acclimatization too!) few woods are as stable
as poplar.

With the proper joinery, poplar will be _less_ likely to split than
an open grain wood like oak, less likely to distort a light frame than
most other hardwoods, and a heck of a lot cheaper than oak.

There's a reason why they make wooden screen doors out of poplar.

If you wanted more strength than poplar, I'd recommend maple (or even
birch), but only if it's been seasoned/dried/allowed to acclimatize for
a long time[+]. And I wouldn't use it where the climate changes radically.

[+] The maple for our plant stand acclimatized for over 5 years. That's
the story I told my SO, and I'm sticking to it!
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.