What about something like this,
MEC Etape Top Tube Bag - Mountain Equipment Co-op. Free Shipping Available

But mount on top tube just in front of the seat tube? I've never used these bags so don't know if they tend to flop sideways.
MEC seems to have other $15 ones but perhaps you already have all kinds of frame bags.
In any case I agree with wanting something very secure with that crucial stuff.