The heater cores are taken from cheap household ceramic heater-blower.
I needed to get the HR-V's stock heater radiator out of the car to measure it's dimensions and to build an adequate substitute. It seemed to be a fairly easy task but I found out that I have to completely dismantle cabin dashboard to access the ventilation box, take it off and get the heating radiator out.
Here is how the car cabin looked when I finally took the ventilation box.
Once I opened the ventilation box I got access to liquid heater radiator.
I placed 2 ceramic heater elements on liquid radiator where the best airflow is expected and marked the dimensions outline of them so I would know their location when building heater elements box.
Next I took 0.8mm stainless steel sheet and built the heater box by marking, cutting, bending and riveting it to resemble the dimensions of stock liquid heater radiator. To hold the elements I used the plastic holders that were originally in electric heater. I sawed them off to take the shape and fit side by side. It fastened them to the box using two bent steel sheet retainers which are riveted to the bottom plane of the box. I connected the heater elements through over-temperature protection switches that were in stock electric heaters. I run a 2.5mm2 wires cable through plastic cable-through holder in the same place where one of water pipes is going out on stock liquid heater. This way the heater cable goes out into the engine bay where the liquid heating pipe used to come in. Here is box view from top.
Must admit that initially box came a bit over-dimensioned because stock radiator had round corners while mine had square ones. So to fit it in I had to grind corners a bit and adjust bends. After that it lost a bit of it's tidy look. Here is the view from bottom.
I then placed the newly made electric heater box into ventilation box. Had to make a bit wider openings in ventilation box plastic where white cable holder goes out.
Then I've assembled back the ventilation box and ran a short test by connecting the fan to 12V battery and heater elements to 220V socket. After few brief moments I had around 50 degree Celsius hot air blowing out - should be ok in a car on a cold day. Of course car's nominal voltage is around 140V so out of two 1500W@220V AC rated each heater elements I should be getting around 1900W. I also placed a small DS18B20 temperatures sensor into the box airflow path to be able to measure the air temperature and regulate the heater element temperature from a control box BCMS.
Then I put assembled ventilation box back into the car and started assembling the cabin dashboard again.