Here's a few bridges to span gaps.

My idea for the bridge evolved as I was creating it, I've included all the designs I made (for better or for worse). There are some images to help people understand what I'm talking about below. Although I imagine none of it will make sense with or without images if you haven't made some of my stuff before. In those cases, hopefully the images will at least give you a clue.

"Bridge [02a]" was the first attempt, it's a 2cube span that's the full 75mm wide. I designed "HR [01]" to go with it. It works, but only on the outside holes. After printing a set, I realised that using the outside holes meant the bridges couldn't connect to a "top" with "walls" at the sides. Also, it meant that a "top" couldn't form a corner with 2 "bridges" because the pegs would clash.

After the corner peg revelation I made the rest of the "bridges" at 70mm so that they would connect to the "tops" by the next hole along (the same used by the bottom of a "wall"). This meant that the previous issues were resolved.

Finally, I designed "HR [02a] & HR [02b]" as a universal fix. Using these "handrails", any "top" can become a "bridge".

I numbered the files awkwardly because "Bridge [01a&b]" will only work with "HR [01]". All the "bridges" will work with "HR [02a]" and "HR [02b]" will work with any 1 cube "top".

"HR [02a&b]" will also work with a half-cube "top". Connect the "HR" to the outside holes of the half-cube "top" and it will sit in the centre hole and the hole next to the corner.

Hope any of that gibberish helps.

P.S. "HR [02a&b]" have been cosmetically altered since the images were made but they still fit the same.