Do you know what temperature it gets to inside the garage? That will make a big difference as to how much insulating etc you are going to need
Are they going to be in hutches or cages?
If they live in cages and it is cold in the garage, then you are going to need to put on a lot more covers etc as cages won’t stay warm. If they are to live in the garage long term, would be worth investing in good quality hutches.
For outdoor/shed piggies, thermal hutch covers, old carpet, tarp etc are all needed to stop cold air exchange and draughts.
As long as the paper bedding is put in very deep. The thing with outdoor piggies is using hay to completely fill the hutch helps insulate and fill the hutch so you will want to replicate that with a lot of paper bedding. Wet bedding needing to be removed daily.
Does all hay give them sore feet? I get my meadow hay from Haybox and it is very soft and makes lovely bedding
You mention towels. If it gets cold in the garage, then I would not use any fabric bedding, particularly not towels. If the piggies pee on them, then they wont dry and then piggies will be sitting on wet fabric.
My two boys live in my shed and I have a thermal hutch cover, then two blankets, then a normal hutch cover and then a duvet thrown over all of it. Inside is then stuffed with hay and i put in four snugglesafe heatpads - two in the bedroom end and the other two spread throughout the hutch. They are put underneath the hay as it helps keep them warmer for longer.
Id recommend you get spare covers for the snugglesafes. I had some made and they are thicker than the ones which come with them which helps them stay warmer for longer (they say they last 10 hours but depending on how insulated the hutch is, how cold it is etc, it’s more like 6-8 hours)