Filament Passage

I've built a housing for my printer out of 18mm thick wood. I wanted a passage for the filament so I've made this. You need to cut a 40mm square into the wood. Then put the top part on top (of course ... ;)) and attach it with 3mm screws.
You can then either put the "filament passage bottom counter sunk" in the hole from the other side and turn it by 90° towards the top part and fix it with 3mm screws too. The other possibility is using the "bottom flat" part and put M3 machine screws through both parts from the top.