T-slot 20x20mm, 3mm nut remix

I have edited the model slighly so its more tolerant in size and will fit 'too' easy, since some of my printers aren't perfect, tolerance helps alot.

Also for my own o.c.d. i have centered the nut in the piece, and removed the 'bottom' to allow to print without support or weird overhangs along the side. also decreased total size.