Pinecone
En-Route
Not sure if this is viable or not but saw one happy owner on the 182 facebook page that is in a situation like yours.
Basically he is using one of the new small portable camping diesel powered heaters. That is then ducted up into the cowl flap from the bottom. The nice thing about this setup is that it will run off of a lithium battery pack so you can just place the diesel powered heater and battery pack on the ramp and let it run with no need for external AC power. I thought that I've read these small diesel heaters can be a bit ornery (as in don't always run all night) but seemed like a great very portable solution. I would think it would also start to heat the cabin as well. Definitely not something you want to build into the plane but seems like it would only take minutes to setup and a Jackery style large power bank should be enough to run the small fan and electronics for a few hours. Could also be used for camping I suppose.
I already posted a similar system from Mooney Space.