I liked the DIY slider top idea, but didn't feel comfortable with 1/8" thick glass. We made ours with 1/4" glass, and a single u channel for the slide.
The u channel we used fits great, is clear, and accomodates 3/16" or 1/4" glass. It's available from Mcmaster.com ( 1753K62 Impact-Resistant Polycarbonate Channel, .365" Base X .345" Legs, .050" Thickness, 4' L, Clear)
The u channel was cut to 4pieces @ 11 3/4" long. We got 2 pieces of glass to fit inside the channel @ 22 7/8" x 6", and 2 pieces to sit on top of the u channel @ 23 1/8"
When installed, the u channel sits on the tank frame, and the lower piece of glass sits in the u channels, to slide ffront to back. The top piece sits above the u channel, but is contained by the edges of the tank top frame. since the glass is cut to 6" there is a small overlap. If I were to do it again, I would cut one of the top pieces to 5", so there was less overlap on the side of the tank that has filter tubing along the back and side.