I would suggest that if you wish to use an extension cord to get the Heavist one you can find. I use one now, not for a heater, but power tools. It is a heavy duty job and I don't get cord heatup. I just stuck mine into the closest wall receptical(SP?)and drilled a hole thru the wall passed the cord thru and then use chaulk to seal the hole. It is a jury rig, but it will work for me for now.
Good Luck

1eyedJack and the Dawg

The best and safest way is to use armoured cable and bury it.
If you do use an extension cable as a temporary fix, make sure its protected by an RCD (RCCD) type device.

My husband did it to codes (buried) with those safety features so I don't electrocute myself. I heat my Rion (8X16') with two small inexpensive $20 heaters and a third cheapo for really cold nights. Two are on thermostats from KKontrols.com. The smaller 6X8' HFGH is also heated by a $20 heater that keeps it cozy on the coldest windiest nights.

The frugal greenhouser.....