My thoughts around how thick the liner would be is to think in the bounds of kevlar vests, which I use often, it gets hot, but not unbearable. But the overall thickness in the kevlar is usually around 3-5mm, so a combined thickness (including the ABS) up to max 5mm might work.
There are probably a million fancy ways to do this mod, and likely most of them will be unpractical, too warm or not possible to make thin enough etc.
I´ll at least be working on this through the year and seeing what I can come up with, I´m in no hurry as this is my endgoal for the completely finished version I want. But let´s keep the ideas coming and maybe a "standard" for what works will be set and people looking to do this mod gets reference and help when it´s their turn:-)
About smooth sprayrubber, I don´t know. But if some exists, it might be worth while checking out though.