Unable to perform boolean substract nor to trim a solid

Please help!

I’ve been working all day long trying to perform an appearently simple operation: a boolean difference. I’ve been reading topics about boolean union and tried to fix things accordingly, but it still wouldn’t work.
I have a structure that should hold a sheet of plastic. The structure has a sort of frame with a tongue and groove profile. I’ve modelled both elements. Now I’ve been trying to make a copy of the structure, to boolean substract it from the sheet, to obtain the intersection shape and profile on the sheet. It would’nt work whichever approach. I’ve even tried to extract the surfaces from the frame element and trim the sheet the profile, and also made closed polysurface (a solid) in order to boolean substract it from the sheet element.
I’d appreciate your help, thanks in advance!

Desk top.3dm (15.7 MB)