Sorry, should mention I dont ride flats. Ive tried, im terrible in the snow lol
Although I havent tried the warmers yet. Im worried about lack of space as it is.
Pick 1:
-Cold feet on SPDs
-Spending money on real winter SPD shoes and shoe covers
-Flat pedals and warm snow/hiking boots
Unless you go to some pretty extreme measures, clipless pedals are a losing battle, the cleat is usually very close to your foot (even on many of the winter SPD boots), which helps draw heat from your foot to the cleat, which when combined with the pedal and the lack of of your foot flexing to force blood into it, means that your feet are going to get cold. In other words, they are great heat-sinks. It's hopeless with summer shoes and even winter shoes at times.
I do ride most of the time with SPDs, down to around -20F, but that was a race where I wouldn't be able to sustain my foot temp over a longer time than I was out, when I want easy warmth or it's going to be super cold, I put my flats on and wear regular boots. Otherwise, when it's cold, I use my Lakes or my Wolfhammers, with shoe-covers, with thick wool socks (preferably merino, I'll spray my feet with antiperspirant to keep them from sweating, sometimes use a thin silk sock as a base layer, my hotronics battery packs will be attached to my boots (the insoles have the heating elements) and I'll put snow gaiters over everything, which eliminates any snagging issues with the drivetrain and also boosts the heat a bit. I'll also stick them in the oven on warm before a real cold ride from my house. I find just the Lake/Wolfhammer boots are usually good down to somewhere in the 20s, although sometimes in the 20s it's great and sometimes they suck. Going colder takes some of the additional methods. I hate cold feet too, but unless you are willing to go to some pretty extreme ends IMO, it's inevitable with clipless pedals.
I've tried the plastic baggie vapor barrier thing, it doesn't stop the heat conduction and heat sink effect. Footwarmers haven't been super effective for me in the cold either. That relatively unprotected front of your boot can be in the -F temps, and you are relying on a layer of thin insulation between your toes and that cold rubber? (this is why shoe covers help) The heat conduction still happens and still sucks heat out the cleat, which makes my toes cold.
When I go running in the cold, I find it takes sometimes a few miles before my feet are warm, I suspect that is just the effect of my body/feet heating up the entire shoe, or at least enough of the shoe that it's not overcome by the contact with the cold surface. Once I've reached that "operating temperature" I'm good, but I don't think the same thing happens with mountain biking, because with running, you are forcing bloodflow each time you strike your heel and rotate your foot up to launch off the ball. It's established that your calf muscles act like secondary hearts while running, to move blood, these are what are actuating your feet though, moving you forward. This doesn't happen when riding, your ankle rotates, and that's about it.