I downloaded the mod from mdbd and edited the ini file, but still don't get full widescreen.

If you're doing 1.03, go to http://1fxmod.org/clpatch/.

Make sure you open \GOG Galaxy\Games\Soldier of Fortune 2\base\mp\autoexec.cfg and add the commands:

seta r_mode "-1"
seta r_customwidth "1920"
seta r_customheight "1080"

For SP, you may have to use a hex editor.

Looks like the 1fx patch is only needed for multiplayer as the info on the PCGamingWiki just says to enter those commands at the console for single player (so an autoexec would work there too).

[url=https://pcgamingwiki.com/wiki/Soldier_of_Fortune_II:_Double_Helix#Widescreen_resolution]https://pcgamingwiki.com/wiki/Soldier_of_Fortune_II:_Double_Helix#Widescreen_resolution[/url]