Remedy for leggy squash seedlings

Morning fellow veg-o-nauts. I sowed some squash seeds on 20th April and the majority have germinated. (result). The problem is that they're starting to look leggy and I don't know what to do so that they're in good condition for planting out later in May. They were started off in an unheated propagator on a south/south-west facing upstairs windowsill. I have nowhere else to put them indoors, other than on the floor in an upstairs east-facing bathroom. The level of the potting compost is quite low in the pots, so I could top the level up, if that would be of any benefit, but really don't want to start again as the seeds were quite expensive and don't want to ditch the little blighters.

Thanks JayG. When I move them back into the propagator, should I leave the lid off, or keep it on and the vent open? Plus, is there any use in topping up the seed compost in the pots to give them a bit more stability?

Ok, JayG, Gotcha. There are two squash seeds in the propagator which haven't germinated yet, so I'll give those a bit longer and move the seedlings forward as you suggest. Thanks so much.

Hi Yorkie - thanks and yes, I've moved the seedlings to the front of the window sill and backed them with a silver foil board. The two remaining squash seeds are still in the propagator - no sign of those germinating as yet.