You can make res files manually, too. They're simple to write.
They follow this format:
// res name / optional comments
// file entries:
custom.wad
sound/weapons/sbarrel1.wav
sound/ambience/siren.wav
sound/ambience/jetflyby1.wav
sound/weapons/mortarhit.wav
sound/debris/beamstart11.wav
sound/weapons/electro5.wav
sprites/muzzleflash1.spr
This would download custom.wad, as well as sbarrel1, siren, jetflyby1, mortarhit, beamstart11, electro5, and muzzleflash1 if it did not find said files. Just make sure to include this res file with your map.