Lagering

Wondering about lagering, (newb), My basement is warming up currently it's 15C (59F) soon it'll be 17C, 63f. Here in Montreal it's still cold at night. 2-5C. (41f and below). I'm thinking to put it outside to lager for a while on underground steps. I think temp fluctuate less but still up and down maybe by 15F Is this no good? If so then I guess alternate is bottle and lager in fridge in bottle? I don't have spare fridge just for lagering?
What is shortest lagering time?

15C is a bit warm, and you really want to try avoid temp swings during fermentation. It might be fairly easy for you to pull off a swamp cooler without much maintenance at those temps though.

Thanks. What is a swamp cooler?

put your fermenter in a big vat or tub filled with water. Then if you need to warm it up you just pour some hot water in or cool it with frozen water bottles. This will help with decreasing temp fluctuation.

Playing with Peltier. Expensive toy so far. Can't get heat away fast enough so the whole thing heats up. Heat sink &fan insufficient. it's 100w can't see it chilling 5 gal. Let you know if I crack it.

Screw the peltier. I hoped I could tape it to carboy and forget it. The cooling of the peltier itself is such a pain, not worth the hassle. New plan is to use an air conditioner blowing into a cupboard. Not sure how low they can go. I feel 2nd hand fridge coming on.

A swamp cooler is what Arneba described above. Short of a second hand freezer or fridge with a temp controller they're the way to go. A cooler that will fit a carboy/bucket or two with a makeshift top of cheap insulation will keep temp pretty constant if you add ice as necessary (or an aquarium heater if you're trying to warm it up).

2nd hand fridge was way to go. Craigslist and $50 gets me 30F. I had to err trim door with scissors and use tape to keep door shut. But I wish I'd done it sooner. Now I need another for the fermenter. this has freezer compartment at top. Storing lemonade there having seen hard lemonade recipe. I think these need to be full to cut ice build up.

I thought a swamp cooler was when you wrapped a wet towel or t-shirt around the carboy, put it in a puddle of water, and pointed a fan at it.

Peltier works much better when it's matched in surface area to the object it's cooling off. Liquid cooling the peltier also helps out a lot. It seems like an over-engineered method to cool a carboy, given the cheaper and easier alternatives.

swamp coolers (with or without tshirt and fan) are great for temperature stabilization, but it's really a stopgap solution. if you have the space for a 2nd fridge/freezer with a temp controller, there is no better solution when it comes to simplicity and robustness of the solution, in my opinion.