I build a "box" with a glass lid once that classified as a cold frame. I put a thermometer in it and noted the temperature inside the "box". I seem to recall the widest swing in temperature was 45 degrees outside and over 100 degrees inside the "box" on a sunny day. It was more of a hassle babysitting the "box" trying to figure out when to prop the top to cool the box down, than it is to start seedlings inside and harden them off outside before transplanting in the garden.
Yeah, babysitting the box can be difficult. But if you have a short grow season it may be time we'll spent. For some, no inside spot so outside protection is next best option. Either way, less cost than buying starts!
I think a temp gauge that sends info to a remote would be my consideration for a more convenient way to monitor. Most days you could just look and go.