Thanks for the advice Max and Normk,
Max, I did all that but still find it very difficult, but that may be because of age and various maladies that come with it. I find if I use a small clamp and hold the whole lot down while I insert the rope, that makes life a lot easier. I resorted to using macrame cord (believe it or not) and find it works great. It is only around 2.5 mm diameter but has the same break rating as starter rope and paracord. Very strong stuff! Makes me wonder why is such strong cord needed for macrame? Haha! It is very easy to work with and I got it through the holes first try. Left over from a relative's macrame obsession way back in the 1970s!
Norm, I'm slapping my forehead. Funny, I never even considered drilling the holes bigger. Yes, that makes sense and would make it a lot easier inserting the wider diameter cord.