Get thee to Zellers, or Canadian Tire or some other housewares store and buy a semi opaque storage bin similar to the one in the picture. Make sure the lid is flat and not curved, as some are. Trick is to use it upside down. Put the seedlings on the lid, fit bin over top and Wa-La!* Instant temporary greenhouse. Any seedlings you have on a window sill can safely go outside in one of these, especially sweet peas — all cool-loving seedlings, like parsley, or perennials.
Warning: Put in shade at first, so seedlings get acclimatized. The switch from indoor light to outdoor shade is huge, and you don’t want sunburn and death to seedlings you’ve been nurturing. Remember to vent your greenhouse if days get hot.
Or, bring indoors in evening, if the night time temperature looks frosty.
I’ve just put my sweet peas and some lavender I bought a couple of weeks ago outside in one of these bins. They were looking a little spindly even in my south facing window sill.
*Credit for this mis-pronunciation of the French exclamation Voilà! goes to one of those K-Tel-like commercials from the 70s. We think it was about home-made wine.