In reply to Sans-Plan:
I’ve been through a lot of ski touring packs looking for my perfect one, and always had a few annoying issues that kept me searching, though I think I now have my keeper: the BCA stash 40. It might be larger than you need but it comes in 2 smaller sizes:
What I like: good large volume, a traditional draw cord top opening (like you I find the more common top zip closure is harder to pack fully and always feels like it’s going to pop at some point), top lid pocket, a zipped back entry to open it up quickly if needed, good avi compartment, 2 decent sized hip belt pockets (a high priority for me), helmet carry, durable but decently low weight, comfortable to carry.
Packs I went through that fell down on features I wanted: Black Diamond Cirque 45 (almost perfect, except the useless single hip belt pocket, and had to buy the top lid separately), Osprey Kode ( usual Osprey over-designed, multiple top zips that always meant the one I wanted was the last I opened, tight for its supposed volume), Blue Ice white Tiger 35 (pretty good overall, but tight volume for me, and I prefer a draw cord to zip main closure), Lowe Alpine Alpine Attack (not specifically a touring pack, not bad but missing the specific touring pack features.)