Potting up - Size, depth question

[Joz]

I've got a pile of seedlings going... started on Feb 4 in a 72cell flat, and some potted up into ~3.5" plastic cups.

Problem now is, they're a bit leggy (some are about 7" tall), but it's still a touch too chilly to be putting them outside (nevermind I've not hardened off yet).

I'm thinking about getting tall skinny (4" or thereabouts) water bottles, cutting the tops off, and burying my leggy seedlings within.

Are there any other brilliant ideas for handling this situation? I have some larger pots, but the depth seems to be the important thing at the moment... and to plant deep enough, the pots are far too large.

Thanks....