Homebrewing - Beer, Mead, Wine, Cider

Last time I brewed at home, I had my fermentation bucket in my flat, where the heating pretty much took care about all thermal regulation I needed back then. As I now have kids, I don't feel comfortable doing that anymore for various reasons.

I have freed up some space in my garage now for brewing & fermenting, but I have no heating there. I'm OK though to go with the seasons, brewing beer styles where the yeast's preferred temperature roughly matches the weather. But now, my mind is occupied with the question of how to keep the temperature as constant as possible for fermentation: While a weather forecast of e.g. 15°C doesn't sound too bad for lager beers, it may easily get as cold as 5° at night, giving the yeast probably a rather bad time. As I also don't want to spend a fortune on a temperature regulated fermenter, I'd like to even out those mins & maxes passively.
My thoughts so far circle around insulation (obviously) and thermal mass. Insulating the bucket itself seems like a nobrainer. But I think it also might work to build some cheap wooden enclosure, insulate that with Styrofoam, make everything somewhat airtight and add water bottles, rocks & bricks to fill up as much space as possible. That will of course do little should the weather change drastically, but so far, I think I'd stay way below max and above min temperature in there at all times. This way, I believe I could get a decent fermentation when the average outside temperature of night & day is right for a couple of days.

Is anybody here doing something like that or has experiences worth sharing otherwise?

P.S.: Addressing the elephant in the room: For now, fermenting under pressure is no road I want to go down. Buying a new fermenter, kegs, valves, fittings, hoses, CO2 bottles and either a counter pressure bottling system or even switching to drafting entirely is just too much right now.