Dissolve gelatin in hot cranberry cocktail.
Add cold water.
Cut ice cream in 6 chunks; add to gelatin mixture and stir till melted.
Chill till mixture mounds slightly when dropped from spoon (about 20 to 30 minutes).
Stir lemon peel and lemon juice into cranberry sauce; fold into gelatin mixture.
Chill again till mixture mounds when spooned (20 to 30 minutes).
Pile into cooled pastry shell.
Garnish with whipped cream and cranberry sauce, if desired.