Grille block and engine belly pan!
Even if you can use the block heater only on half of the trips, it's still worth it. Evenings may be cold, but mornings are even colder, and with enough insulation there might still be some heat in the engine bay after 4 hours. Also, there is always a chance that you'll find a plug somewhere (parking lot, friend's house, etc.).
You could look into things like a coolant thermos (gen 2 Prius) or using exhaust to warm coolant (gen 3 Prius), we'd all benefit from your experience and a detailed how-to with pictures
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread