Hydraulic Porta Power 

Our timbers are #2 and better. They are 6" x 6" spruce, pine or fir. We notch them on site and fill the mat.  
Here is how we get a tight pack. We use a porta power hydraullic jack and tighten the pack in each section.
We measure the void and cut a 6"x6" insert down to snuggly fit the void.
The last insert is cut to size and placed in the void. Pressure is released from the jack. We sledge the insert into place.

Note: Rig mats with inserts placed crossways are very simple to repair in the event of damage to the wood.