how to install a chainguard, when the chainring doesn't have screw holes? I'd like to install a chainguard like this:



on a bicycle that's will be spend it fair amount of time commuting, but on closer inspection I see that the largest chainring doesn't have the necessary screw holes, to my surprise (naively enough I assumed this would be "standard").

Also, since this a Hollowtech crankset, using something like this seems to be out of the question as well:



Is there a way to attach a chainguard to a bike like this, without having to resort to a full-size chainguard?
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'.