I found two place online where someone actually photo documented their sofa reupholstering project, here and here. I was amazed that in this current DIY culture that reupholstering instructions were not easier to find and believe me, I tried!
I did finally sew the adjustments I pinned into my store bought slipcover way back when I started this blog. I altered the arms and it makes a HUGE difference that they now fit. I was able to keep the piping in place so it looks as if it was originally made to fit. I have a small loveseat and I bought a slipcover for a slightly larger one so it still has too much length from arm to arm and the elastic they put in back for adjustments just bunches up. I will have to take it off again, turn it inside out, and pin some more adjustments for those changes. However, with just this small change, it fits better and doesn't slide around as much when in use.
I also seriously looked into prices on foam for the one-piece bench cushion I want. I entered the dimensions (44" x 26" x 4") into the appropriate forms on FoamOrder.com and FoamSource.com. They each offer different grades of foam in three densities. Since this is going to be a temporary solution I selected the lowest grade and a medium density foam.
Foam Order's quote came to $68.85.
Foam Source's quote came to $63.24.