My reptile room ranges from 79(night) to 84 (mid to late day) for the warmer parts of the year, 72-84 in the cooler months, and I have kept many snakes, including some garters, at nothing but ambient over the last few years. I currently have valley, mountain (elegans), infernalis, ordinoides,and checkereds living without additional heat, and they are doing great. All I have noticed with the belly heat is that they get ravenous appetites instead of just great.
I am slowly but surely getting every cage over 4" heat tape to give a warmer belly heat spot, but even without it, they seem to good at those temps. Pointing a lamp with a lower wattage bulb at the outside corner of one end, as you said, should work fine for temporary as long as you check the temps after several hours to make sure it isnt getting too hot. Also need to make sure that no one else in your house, or any other pets, will accidentally knock the lamp over or closer to the cage. I would think a 75-80 gradient would be fine for a while. Night time temps can go much lower with garters than many exotics, as valley garters are typically found in areas that get pretty cold at night even during some of the active months. Just make sure they get several hours of heat a day to digest food and prevent them from slipping into brumation unless thats what you want.