No need for a timer, just keep a little heater at a low setting that will pop on when the temp gets low.
Assuming you have a thermometer (preferably with memory) you can just trial-and-error tweak the heater so it keeps the plants optimum at night. The temps are fine during lights on, so the heater has no need to heat.
I use a similar low-budget heating strategy, but kind of in reverse, heater comes on with lights to maintain 78F -ish because I am using LEDs and they don't make much heat, I am trying for a 15 -20 degree F drop at night, works well usually, but it has been near freezing out so I have been getting into the 50s, somewhat concerning.
58F isn't too terrible; mid 60s are better for night temp.
The worst that can happen is that you will stunt growth and miss out on super sweet nugs of crystally wonder.
The best thing would be to get a day/ night temperature controller with adjustable temp. settings for day and night temps. I think they are like $150ish.
Either way a cheap little heater is what you're gonna need.
hope this helps