Once you have it built and sealed as tight as you think is needed, get a caulking gun full of black roofing tar or other black sealant. Put a comfortable chair and a glass of ice tea or other preferred beverage in your darkroom. On a bright sunny day go in, having left all the lights on outside the room plus the sun on any outer surfaces. Sit in the chair. Sip on the tea. While away 5-10 minutes to allow your eyes to adjust fully to the dark. The older you are the longer it will take. Then carefully look around. Move around in the dark to spot holes that you might not see in the chair. Everywhere there is a bright spot you need to caulk.
I found this a wonderful way to cool off near the end of the building process. It may take more than one visit to accomplish what is needed. It will be well worth it every time you have undeveloped film out in the darkroom.