Killing Ferdinand VII would be a big help. Not just in making Span a Constitutional Monarchy, but in general as well.
I do agree with you on that aspect. He was a coward, selfish and self-centered ruler. The fact they brought him back after the British and Portuguese drove back the French out of Iberia would hurt Spain very much. Maybe a good way to kill him would be while he is imprisoned at the Chateau of Valençy in France. Have him die captive there before August 1808 before the Council of Castille reversed the abdication.
Alternatively have the French win the Battle of Bailén. That was the key battle that proved Spain could resist the French occupation. Either a French victory or a not so decisive Spanish victory outcome would lead to the Council of Castile not deciding to reverse Ferdinand's abdication.
By the way if we go with the first scenario considering he had no heirs at the time the throne would pass to his younger brother Carlos, Count of Molina. He was the fist of the Carlist claimants and a very staunch absolutist. So to get a Spanish Constituional Monarchy you also have to get rid of him. He was also a prisoner of Napoleon as well so you might wanna have him also die in prison too. Your best bet for a Liberal King out of the Spanish Bourbons might be Infante Francisco de Paula. While he is a bit conservative like his older two brothers, he is more of a moderate liberal so he might be willing to keep the Constitution of 1812 if he does become King.