building a mini root cellar

Not sure if this is the right folder for this, but here goes. If we are even remotely successful with food this year, especially potatoes, we will not have sufficient storage. I want to build a root cellar. This would be primarily for storing root vegetables and fermented foods (sauerkraut, etc) through the winter, and would probably not be used much in summer. In the future I'd like to build a big walk-in root cellar, but that's a massive project so it's in the future. For the moment I am considering something smaller. We have a corner where the garden has been built up steeply, there is a set of steps built with concrete block sides and railway sleeper steps, and a 2 foot high concrete block wall at the front, then kind of a hole behind that rising to a steep slope up to about 6 feet above the level at the front.
I was thinking of either burying a couple of barrels or an old fridge or chest freezer - putting it in the hole and backfilling. That would be super easy to do and cheap, but might not offer everything we need. Another idea is to build a small concrete block bunker into this hole. That would be more expensive and more work, but would offer more space, easier access, and better security. I'm thinking of concrete block floor and sides dug back into the slope, and covered with a green roof, then having basically shed-type doors but insulated with 6 inch insulation boards (celotex). I'm not a huge fan of using concrete blocks for lots of reasons, but this might a case of 'needs must'. As in, we will really need something by around September and I'm just not sure that other options would work. I am open to suggestions though!

Our winters are not that cold (rarely below -5 C) and our summers are not that hot. The other major considerations are ventilation, waterproofing - it's WET here - and vermin protection. I'm a little worried on balancing those. If I go with the bunker idea, I could put air vent pipes up through the roof, but I'd need to rat-proof them somehow.

I'd be really curious to hear from anyone who has built a successful small root cellar.

Ventilation is super important.

Pretty much the only way you can expect much success is to make it easy to check on your vegetables regularly. If you're not checking in regularly, you'll need fans and thermostats, etc. Better to just go look every now and then. I'll be checking mine every few days until I get a feel for my new root cellar. So I'd recommend making it as accessible as possible.

Thank you. That's a good point. I had been thinking about making it accessible in terms of just going to get veg when I need it, but the need to check on it as well means the barrel option is pretty much out. I can't imagine wanting to go check that every few days in the winter in the pouring rain...

There are lots of buried-garbage-can designs out there but I'm completely skeptical. My parents did manage to keep some root vegetables over one winter but getting anything involved shoveling off a sheet of plywood and getting down in the hole. The first few attempts were successful but later they went back and found everything frozen. They figured the straw wasn't put back right or the plywood didn't contact the ground all around, causing heat loss. Anyway, my point is still accessibility. Obviously, you have to balance that with cost and time of construction, and other constraints per your situation.

I encourage you to try some kind of root cellar, even if it's not perfect. I started with a shelf in a tuck-under-garage type house. It worked and was a powerful example of the advantages of working with Mother Nature.