Gonna need somewhat of a custom pizza shape for this to work without arousing suspicion. Put the pizza between the toilet rim and the seat. If it sags a bit that's fine, the seat should hold it in place. Print out a picture of the inside of the toilet bowl and place it on top of the pizza. Close the lid.

After a few days, invite the crew over for beers. Rig the bathroom light so that it's very dim. "Sorry, been having issues with it, not sure what the problem is." Eventually someone is going to notice the ruse. When they do, "Oh shit, my pizza! I was wondering where that went." Bring it back into the kitchen and offer everyone a slice. They will refuse. "More for me then!" Eat the whole thing.

Instant legend.