Fooled around with ideas last year, biggest issue, in my mind, was trying to keep the heat in the cowl regardless of which external heat source you used. Cub cowls are hard to plug up.
Commercially avail cowl covers are pretty bulky and heavy. Couple of us started using Mylar space blankets taped around the cowl during heating, this evolved into sewing up a cover out of Mylar tarp which is a little more durable but still light and compact and reusable, attached over cowl with fastec buckles. Cotton pant leg sewn in the bottom to accept heat tube.
the heat source was derived from an MSR stove and some Ace hardware parts.
all cheap and compact. Ducting is 3" aluminum vent hose, (not dryer duct hose) that accordions to about 4 times its collapsed shape. all the components "nest" together when stored
Im still a little nervous around this setup as heat source is open flame. Fire extinguisher close at hand and don't leave heating plane unattended.
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