Boolean union dousn't work

Hi guys,
To 3d print a detail of my rhino model I have to boolean union the things you see in the attachment, I tried everything but the solids don’t join or boolean union, can somebody help me out ?

voor 3d print.3dm (509.2 KB)

Your model has many flipped normals and open surfaces. Some of your surfaces are 2 dimensional. Set your display options to show back faces in a different color and the flipped normals will be obvious. The vertical rectangular shapes are not complete surfaces and have faces missing.

Hope this helps.