Bring 6 cups water to a boil in a Dutch oven; add shrimp, and cook 3 to 5 minutes or just until shrimp turn pink. Drain; rinse with cold water. Peel shrimp; devein, if desired. Coarsely chop shrimp. Toss with lemon juice.
Combine butter and cream cheese. Add mayonnaise and next 6 ingredients; fold in shrimp mixture.
Spoon into a lightly greased 4-cup mold or an 8- x 4-inch loafpan; cover and chill 8 to 10 hours.
Cut baguette into 1/4-inch-thick slices, and place on a baking sheet.
Bake at 350° for 3 to 4 minutes on each side. Cool on wire racks.
Unmold pâté, and spread on baguette slices; top with cucumber.