Hmmm, not really a lot of "free" in that area (I know it well). There are the Walmarts, but they have gotten pretty bad in recent years. When I am in Longmont, I stay at the Moose Lodge (you have to be a member... $70 a year + $10 per night) or at the Fairgrounds ($25 per night with one utility).
There really are no free places in the foothills forest. You have to go all the way up the mountain to Nederland-to-Ward area and as you can imagine, they are overrun with others wanting to *live* there for free (and trash the place) and most are vagrants who you would not want for neighbors. Here are a few:
https://www.campendium.com/west-magnolia-campground
https://www.campendium.com/forest-road-328
https://www.campendium.com/beaver-reservoir
I have stayed at all, and all are fairly equal. Only West Magnolia will get just a tad of Verizon signal when camped at the first few sites, which are usually taken by folks who pull in to them in May and do not leave again until October of every year. Sadly, because of the proximity to Boulder and Denver, the free camping options are either gone or overrun by the wrong type of folks.
If you do not need cell signal, Beaver Res is probably the best one. If you are lucky, and the 80's class-A that lives there year round has moved (it's been there year round for many years now), you might get the spot near the res which has ample space, solar, and access to the water.