Okay, Bic mini added.
IMO stay with the Heatsheet you have (esp. if it's the 1-2 person size). It's much more versatile, and you can control moisture/ventilation. I don't like bivvy sacks, having first sweated and then frozen in one (not an AMK, but an impermeable bag is an impermeable bag).
I have the 1 person one. I looked at the scale and the 1~2 person is only a foot longer but 50% more expensive, so I went with the cheaper one.
No room for toilet paper unfortunately, have enough room for small bits and pieces but that's about it.