We also had no shoes while we were walking uphill to school both ways

My suggestion is to grab a cheap propane heater at Lowes or Home Depot and git 'er done.  I use them all the time for work.  We routinely work in buildings with no heat in the winter.  Some scrap carpet works great for the floor.  Commercial grade is best because you can roll right over it with a creeper. 

It definitely sucks working in the cold, but I find if I wait for spring I end up too busy with yard work, etc. to have time to spend in the garage.  Plus, I want to drive the car at that point not work on it. 

15 degrees is tough to battle.  Learned from my father years ago to use cardboard to lay on a concrete floor, not ideal, but does help a lot. Plus, there was a bicycle store near us that offered a good supply chain.
Agree w/TKO, one of those propane heaters puts out a lot of heat quickly.
 

2nd best thing I put in my garage was a used pellet stove.  On a cold garage tinker day I make sure it is full of pellets and turn it on first thing in the morning.  After breakfast and caffeine the garage is a nice place to go work.  Stove was free off Craigslist, most expensive part was the flue piping.  The heat is dry, unlike a non vented natural gas or propane heater which will raise humidity.  Only catch is that it is “solid” fuel so your local building department may or may not think it is an okay idea.  My local building department made me get it off the floor and draw combustion air from outside (because of the potential of gasoline vapors in the garage).

And there is nothing wrong with cardboard on the floor to insulate you from the concrete.
 

Don't forget the '....impotent fact'...we are a coupla years older and the cold seems to be colder now (and the hot is hotter) You could just say we are  ruin't  like most 'Mercunz'!!
Ain't this a GREAT country!!!!.
6sal6

The two ton mini split in my garage works great, and is now also heating the game room through a 3’ door, since the game room mini split is kaput.  I have been working installing wainscot in the game room.  Foundation was poured Feb 6, 2020 and nearing completion soon!  Then I can work on the Mustang and truck.  Or, shoot pool...  hmmm...