I have a few different ways of starting things - including the jugs outside, heat mats and shelves with lights surrounded with those reflective blankets (a few bucks at Walmart) in the garage and some seeds I'd even put in the pot they'll grow in and clothes pinned floating row cover over the top (my big pieces for the beds rip in the wind - so I'd cut some up). It won't keep the pots that much warmer but will keep the birds off say - the Patio pea (4 inches!) seedlings I grow on the back porch (and sorry, I eat them - don't save seed, buy each year or two).
This is reminding me I need to check the thrift stores for a crock pot (won't use my good ones). With shelving in the garage - one thing I've seen folks do is put a crock pot with water on the bottom shelf on low - warms up the shelving unit if you have it enclosed. You do want the kind of shelves that are not solid - this is either a HomeD or Costco hefty but inexpensive plastic one (and those are the older pruners I need to scrub, sharpen and oil - or toss - so ignore them).
The other thing I do is put more soil in my jugs than a lot of people / sites do - that way if I end up planting out later, the plants are ok and not root stressed.
And, like most here - I start A LOT.
After planting, the worst is carrying them in and out for hardening off. My poor back (all the heat mat / light inside ones). I grow for our Master Gardener sales in addition to myself (MGs here are non-profit, we participate in a school garden in every school in our unified school district and do lots).