how to install a chainguard, when the chainring doesn't have screw holes?
The obvious mounting points for a chainguard are the chainring-spider bolts. 

There are many 'bash guards' available that mount like an extra chainring that are intended to protect the chainring teeth, but would probably do just a good a job of keeping pant legs away.

Most are designed for small MTB chairings but larger ones seem to exist with road -bike BCDs. Try googling '50 tooth chainring bashguard'.