I used a couple of rustic pine 1x2 furring strips, and this plan from Knock-Off Wood (which is a crazy-inspiring site, and one of the main reasons I wanted to buy a nailer and compressor for my birthday).
So - this was one of those frustrating projects where nothing seeemed to go right. I won't go in to the details, except to say that there was absolutely nothing wrong with the plans - I just made several mistakes. Luckily, 1x2s are really cheap - and now I have scraps for future projects (or even more, smaller frames).
So, here are the options:
By the way, these are a lot less subtle in person. At least, I think so.