Something like
this one should do the trick, it's similar to one of the ones I have (mine was picked up in a second hand store for €6), since you can have that to "blow" around the sides of the tent and pot so you're not directly blasting onto the plant.
The room I grow in, my utility room, is a tad bigger than your tent, 4'5" x 5' x whatever-the-height-is, and I have constant temperatures of around 75°F using an older version of that heater on "low", 1kW, setting. That heater is down at low level, it blows across the side of the pot and on the opposite side a couple of feet higher is a fan blowing back over the plant so the heat goes round the pot and is then sent back, giving the circulation around around the plant.
Of course, I don't have an extract system as I don't need it so I don't have the losses of hot air being taken straight out but, ultimately, you're putting the heat where it's needed and warming things from the bottom upwards instead of praying the heat from a tubular goes up, over the top, and down the other side as convection currents carry things around.