Effective diy system for my feet with my spd mtn bike shoes:
At around 5c or so, my mtn bike shoes and MEC rain booties to cut the wind aren't warm enough, so one day I figured I just needed more insulation.
Took a fleece neckup, layed it on the top of my shoe before putting rain hottie on and voila warmer feet.
Would really have to use proper boots for colder temps but this, plus thick wool socks and keeping shoes loose is good for my one hour rides or less when I commute.