I have made and tried some of the 2L soda bottle SIPs, but I dont drink enough soda to keep up with the demand!
Here are two other similar designs I have been trialing. The first uses 16.9 oz (500ml) water or ice tea bottles, constructed just like the above 2L soda bottle SIPs. I reduced the size of the wick to 5in x 3/4in, split in half to almost the end. I did not use the bottle cap however, but cut an inch square piece of plastic bag (my MG potting soil bag), stabbed a small slit in the center of the plastic, threaded the wick end thru it, and put in in the bottom of the upside-down bottle neck. This does not fit tight, and allows a little more air to go up thru the bottom of the soil chamber. I used my knife to make a series of 6 or so small slits in the bottle 1.5 - 2.5 inches up form the end of the soil chamber for more aeration.
The second design uses 2 plastic drink cups, either 32 or 44 oz size, stacked together. I put a soda bottle cap in the bottom of the outside cup to separate them slightly - it makes a small air space around the edges and increases the size of the reservior slightly. I used a knife to make a slit at the bottom of the upper cup to feed the wick through, and several vertical slits around the middle section of the upper cup for aeration. For these I used a 6 in x 3/4-1 in wick, also split down the middle to almost the end. Making this design is so fast and easy. In the future, I might try to get an opaque outer cup the same size as the inner one to block light to the roots, but I really like the clear inner cup to get a look at root development and more essentially, the hydration of the soil in the cup.
Initially I started 25 cuttings in Root Riot cubes and as soon as I saw either roots or leafing out of the cutting, I moved the cutting to one of the above SIPs. I used 80% MG Potting Mix / 20% Perlite, with 1/2 cup of Espoma Plant-Tone mixed in 16 qts of the MG. The potting mix was made slightly damp. Since planting each rooted cutting into the SIP, I waited a week then have added only a little water to the reservior of each - 12ml for the smaller SIPs, 20ml for the ones in drink cups, with any water not sucked up by the wick to the mix dumped out after the first day. The cuttings have been kept in a clear tub with a bit of water at the bottom to keep the humidity up, the tub is covered with clear plastic wrap that I open briefly each day. The potting mix is dry at the top of all of the SIPs, moist at the lower 2/3. I have also transferred a few cuttings started with baggie/paper towels to these SIPS. So far all look very good. I have a few cuttings that I am trying to just root in them as well.
I know the smaller sized 500ml SIPs are only very temporary, and am waiting to see how the figs do with the transfer to a larger pot, hopefully will be able to very carefully remove these without too much root damage. I like this system as it seems to allow me to very closely monitor the moisture of the medium, which seems to be the biggest problem in rooting/growing from cuttings.
Both cuttings were started in the Root Riots on 1/6, transferred to the SIPs about 17-18 days later, so have been in the SIPs for about 2 weeks.