Actually DNA, I've never permanently put together my setup. I had an 8' 2x4 that I cut this way:

(for all measurements, multiply by 2.54 to get the metric equivalent in cm)

2 pieces 4.25" long

The remaining piece I cut in half.

Then I simply laid the pieces out in a box (well, rectangle), laying each piece flat (as opposed to standing them up on the short side).

The 4.25" pieces are only .25" longer than my putter - this left only an eighth of an inch on either side of my putterhead -> this made it so that I had a very, very small margin of error. Any deviation from the line resulted in a knock.

If I was to build one, I'd buy the following:

A piece of wallboard (similar to 1/4" mahogany)

and then I'd screw my current 2x4 pieces to the 1/4" mahogany to keep them in place.

In fact, I'd only keep the 4.25" pieces on the board until I got the longer pieces (~4' long) screwed down - then I could use the board to putt with at distance.

The other nice thing about this setup is that you've always got a straight edge to check your stance to.

(Sorry, DNA, I've got no pics of this thing, but it's pretty rudimentary.)