Really nice sled! You did a great job. Mr NG's 5 cut is very well thought out by him and accurate.
I have used many HDPE runners for jigs....I f they are a little loose tighten them the HDPE expands a little and fills the gap.  If its too much use a file on edge to adjust.  I get the HDPE at Sams, in the form a 24" chefs cutting board and then run it through a drum sander as lightly as I can while still moving to take off the bumpy surface it came with.  I got that from somebody on the old LJ's site and it has served me well.  I did get the Rockler runners for my last crosscut jig I made though, for durabilty.

Mike