One of the fundamental laws of photography is that all enlargers leak light. My Omega DII leaks like a sieve. My old Durst F60 was only marginally better.
As others have suggested, start by painting the wall behind the enlarge with flat black paint. Also the ceiling over the enlarger. It may also be helpful to erect walls on either side of the enlarge (painted black) to contain the leaks.
Also, fiddle with the enlarger to see if you can make some of the leaks go away simply by the way that things fit together. Don't block ventilation holes, but do make sure that the negative tray and filter drawer fit in snugly. It may help to make cardboard flaps that can be taped over fittings that leak while still allowing the fittings to serve their intended purpose.