I am so pleased with how my etrog came out! You can see it here next to our real etrog, which I used as reference--I meant to make the crocheted one about the same size, but as you can see, it came out bigger (which is pretty good, actually, because when it comes to etrogim, bigger is generally better).
I'm putting the pattern under the cut...except I messed up somewhere along the way, and I'm not sure where/how (or even how many times), so the pattern doesn't quite match up to the finished item, and you may run out of stitches to decrease before working all the decrease rows at the end.
When decreasing above a double crochet/front post double crochet stitch, work the dc part of the stitch as a dc and the FPDC part of the stitch as a FPDC. In subsequent rows, continue working those stitches as FPDCs.
Starting in green:
Rnd 1: sc 4 in a magic circle (4)
Rnd 2: (sc 1, sc2in1) around (6)
Rnd 3: (1 sc, sc2in1, 1 sc) around, switching to yellow in the last stitch (8)
Rnd 4: (1 sc, sc2in1) around (12)
Rnd 5: (1 sc, sc2in1) around (18)
Rnd 6: ch 2 (DOES NOT COUNT AS A STITCH); (dc2in1, dc 2) around (24)
Rnd 7: (FPDC, dc, FPDC, dc2in1) around (30)
Rnd 8: (FPDC, dc, FPDC, dc, dc2in1) around (36)
Rnd 9: (FPDC, dc, FPDC, FPDC, dc, dc2in1; FPDC, dc, FPDC, dc, FPDC, dc2in1) around (42)
Rnd 10-11: Crochet around, double crocheting into dc stitches and FPDC into FPDC stitches (42)
Rnd 12: (dc/FPDC 4, dc/FPDC2tog) around, crochet dc stitches into dc stitches and FPDC stitches into FPDC stitches (36)
Rnd 13: Crochet around, double crocheting into dc stitches and FPDC into FPDC stitches (36)
Rnd 14: (dc/FPDC 3, dc/FPDC2tog) around, crocheting dc stitches into dc stitches and FPDC stitches into FPDC stitches (30)
Rnd 15-16: Crochet around, double crocheting into dc stitches and FPDC into FPDC stitches (30)
Stuff lightly enough that the stuffing doesn't show through too badly
Rnd 17: (dc/FPDC 1, dc/FPDC2tog) around, crocheting dc stitches into dc stitches and FPDC stitches into FPDC stitches
Rnd 18: (dc/FPDC 1, dc/FPDC2tog) around, crocheting dc stitches into dc stitches and FPDC stitches into FPDC stitches]
Rnd 19: (dc/FPDC 1, dc/FPDC2tog) around, crocheting dc stitches into dc stitches and FPDC stitches into FPDC stitches]
Rnd 20: (dc/FPDC 1, dc/FPDC2tog) around, crocheting dc stitches into dc stitches and FPDC stitches into FPDC stitches]
Rnd 21: hdc, sc, sl st, fo
Close, leaving about 12" of tail; do NOT weave in the ends (yet)
Pithom: Insert the hook across the end of the etrog you just closed. Sl st. Ch 3, 1 treble crochet, switching to brown. Ch 1 and turn; sc. Weave in ends.
ALTERNATIVELY you could ch 2 and work one dc instead of the treble for a shorter pithom.