I didn't want to drill any holes. In my planning stages I was going to. The best area would be to drill out through the side. As one member suggested, drill from the inside, but only drill far enough to break through the wall. Next use a wood dowel, insert it in the hole, and poke around through the insulation to make sure there are no wires of coils. If none are found, continue to drill until you come out the exterior wall.
I'm using a 950 - 1800 watt ceramic heater. I've never saw the tubular greenhouse heaters. They look like they would do the job, I need to search to see if they are available here in the States. The single 60 watt may be too small to bring the cabinet up to 80°F - 95°. You may have to get one that is 100 watts, or hook up two 60 watts. I found that my singe-stage Ranco does a good job in keeping my cabinet within 1°F.
I'm struggling with the humidity and I'm about ready to purchase a dehumidifier. Finding a two-stage humidistat is difficult, and the one I found, it is better to purchase two single stage hemostats that are capable of control one device at a time. I'm hoping that one humidistat can control the dehumidifier accurately enough that the humidifier would not need to turn on.
I have two air flow devices, and only use one at a time. One is a portable variable speed fan that runs on batteries or AC adapter current. I have also put together a computer fan and hooked it up to a universal AC adapter, and control the speed by selected the desired voltages. Using a fan control will also work, one fan should be enough. If you have too much air circulating, even in a high humid environment there is a possibility of dry the outside of the sausage too fast.
Keep us posted on your progress.