That is a good idea and it's been done: The II gen Toyota Prius has a 3 liter thermos which stores engine coolant, keeping it warm for quite a few hours. A DIY version isn't easy, but is possible. Check out these threads:
Heat storage system (Prius-like insulated coolant reservoir)
Keeping Engine Coolant Hot
Mimicking Gen3 Prius Exhaust Heat Recovery
The last thread is about a different approach to quicken warmup times, used in the III gen Prius.
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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