If you intend to play the game outside of the 4 most common aspects (such as a Bezel Compensated resolution in Eyefinity or Surround) then you will need to create your own custom mod.
The config files that we need to edit for our custom mod need to be extracted from the pak01_dir.vpk container file located in "Steam\SteamApps\common\left 4 dead 2\left4dead2\pak01_dir.vpk" using GCFScape.
The files that need extracting and then editing are:
Now we have our config files we need to edit them using the WSGF HUD Editor online tool to help automate this process. With the online tool enter in your custom monitor configuration including any bezel compensation you have applied. Browse for the extracted .res files and upload them to the tool one at a time. Upon each upload, you will be presented with a drop down menu and a form. From the drop down menu select the profile for the game and file you uploaded.
This will automatically populate the form ready for you to submit for editing. Once submitted you are able to download the edited .res file.