here is my short post on the installation of 4kW Planar diesel air heater:
https://www.trawler.life/diesel-air-heater/
It's the best option I found considering the cost, quality, and time required for DIY installation.
I used 5" or 6" PVC rigid pipes to route the air from the heater to the cabins and heads and only last 2-3' were done using flexible 4" heater hose to minimize the*heat loss to the engine room.
very happy with the results after almost 24/7 use while cruising the coast of Maine and Nova Scotia in May and June last year. One 4kW heater was enough to keep all cabins including two heads on 41' trawler*comfortable.