I used to work in a field that used \Sigma for covariance matrices, and pervasively needed discrete summations which also use \Sigma (and often with an understo

I used to work in a field that used \Sigma for covariance matrices, and pervasively needed discrete summations which also use \Sigma (and often with an understood index set, so the \Sigma appears without clarifying adornment). I ended up writing my discrete summation \Sigma's with a little serif on the bottom, and ordinary \Sigma's as in OP, with 4 quick back-and-forth strokes.