So, I was having issues with my knife block - it used to have many long black plastic bristles which worked fine for a while but after they get damaged it can make inserting a knife quite annoying. Rather than buying another block, I thought it would be a good idea to make an insert instead.

There is an outer lip to keep the thing from falling further in, the dimensions are approx:

Top outer lip: 120x100
Inner Bit: 107x89

It fits perfectly for my block - yours may differ. The knife holes are as follows:

70x6
60x6 (2)
40x6 (2)
30x6 (2)
20x5 (7)

Depending on the width of you knife handle, you may have issues using the smaller knife holes.