Line an 8 1/2-×4 1/2-in. loaf pan with 2 large pieces of plastic wrap, leaving a 3-in. overhang on the short sides.
Beat cream, sugar, lemon zest, and lemon juice in the bowl of a stand mixer fitted with a whisk attachment on medium speed until starting to thicken, about 1 minute. Raise mixer speed to medium-high; beat until medium-to-stiff peaks form, 3 to 4 minutes.
Fill loaf pan with whipped-cream mixture, stopping 1/2 in. from top of pan (you'll have about 2 cups remaining). Transfer 1 cup of the remaining whipped-cream mixture to a small bowl; cover and refrigerate until ready to decorate. Reserve remaining whipped-cream mixture in mixer bowl.
Wedge 1 1/2 cracker sheets into whipped-cream mixture in loaf pan, with crackers placed upright on their long side running parallel to the long edge of the pan, and positioned about 1/2 in. from pan's long edge; this will create 1 “cake layer” in pan. Push wedged crackers deep enough into whipped-cream mixture so that they are mostly covered. Repeat process with remaining cracker sheets, making 6 cake layers total, each spaced about 1/2 in. apart.
Spread cake top with reserved whipped-cream mixture in mixer bowl. Wrap loaf pan in plastic wrap. Refrigerate at least 6 hours or up to 12 hours.
Unwrap pan. Place a rectangular platter on top of pan, and flip over to invert cake onto platter. Remove pan, and carefully peel off plastic wrap. Spread reserved refrigerated 1 cup whipped-cream mixture over top and sides of cake. To decorate, grate zest from whole lemon over top, or garnish with candied lemon peel, if using. Cake may be lightly wrapped in plastic wrap and stored in refrigerator for up to 3 days.