The only space available for placing the pickup is the area between the end of the fretboard and the bridge. Lap steel guitars sound best when the pickup is placed close to the bridge, thus giving a brighter sound. You can determine the best position by ear. String up the guitar and try to find points with rich harmonics. You probably know that one point with distinct harmonics is the halfway point of the scale. Gently tap a finger to a string at the 12th fret while plucking it. You'll hear the harmonics right away. Now do the same close to the bridge; it's more difficult to produce and hear the harmonics there. Try at different locations on the bass string (6th string) and note any occurences of harmonics with a pencil mark on the guitar plank surface. I discovered harmonics 9-31/64" (241mm) from the 12th fret which where also noticable at the same location on the fifth and fourth string. Remove the strings and scribe a "harmonics line" across the plank at the chosen point.
Next, put the single-coil pickup on the plank and align the centers of its polepieces exactly above the "harmonics line". Then lightly pencil the pickup outline on the plank. Add 1/32" (0.8mm) to all sides and draw the required pickup cavity shape onto the surface.