Strap brace on underside of rafters?

I would like to add some additional bracing to the roof on my build however I have already installed the sarking and battens. Do you reckon installing strap brace on the underside of the rafters would give a similar level of bracing as installing it on the top or would I be better nailing some structural ply sheets to the underside? I have already done what was specified by the engineer however I'm not 100% happy with some sections of the roof so would like to brace them more.

Before anyone points out the obvious, yes I am an idiot for only installing the minimum requirements.............


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