I didn't realize you have the Duo inner coming - I was referring to the solo inner.
My solo inner (MYOG from another BPL'er) has LineLoc3 tensioners on the corners. I've using that with 2mm guyline (not shock cord) but I'm sure some fixed length shockcord would be good too. My net inner guylines have clips on the ends, which go to the grosgrain loops at the 4 corners of the mid so they can stay attached when the tent is taken down. During bug season the inner is always clipped inside the mid and then setup is just staking out the 4 corners of the mid (or more in bad weather) and the inner is already to go inside.
The clips I'm using I got from Quest Outfitters for about $0.25 ea. There site seems to be down right now, but they're like this black clip: