The existing template mastodon.txt.tmpl shows a range of daily values. There is now a summary template DATA/mastsummary.txt.tmpl to display yesterdays values.
It's hardcoded to post at 9 a.m. to work in with the rain totals supplied from since.py.
The magic value is in wxtoot.py as self.summary_time if you really need to change it.
Backed off the missing file checks. Should be a little more friendly to use, although slightly more chatty.