I'm getting married soon. One of the decorations is a custom built arbor 7.5 ft tall, 7 ft wide, and 2 ft deep, made with 2x4 legs. After the wedding we want to install it in our garden as a rose trellis. The location we're planning on has about a 1.5 ft drop over the 7 ft width with a shallow drainage swale down the middle of it, although it's more like a yard because it rarely has running water. The ground is hard packed clay with grass over it. How can I install the arbor so it's free standing?
Since the legs are only 7 ft long, I don't want to bury 2 ft in the ground. I'm also not sure if the wood is ground contact rated. It's stained with oil based deck stain (2 coats) but likely isn't pressure treated.
If it were a deck or similar wide structure I could put it on blocks but at 7. 5ft high and only 2ft deep it would be top heavy and fall over in high wind.
I've seen ground stakes for 4x4 posts, but not for 2x4's.
Other than cementing 4x4 posts in the ground, with the low side longer for leveling, then screwing the 2x4 legs to them, how could I install this arbor?