Tyvek, HD trash compactor bags, good ccf pads *can* work if already carried, some use silnylon but the hydrostatic head can be an issue.
Tarp wise i would opt for silnylon whatever size you go. It is somewhat established in the fabric trade that a PU coating weakens nylon fabric whereas sil coat strengthens it, i can rummage for the articles discussing it if you like when i get time later. This is why PU fabrics tend to be thicker, although lighter ones are popping up as per the new lightweight dd. In effect for equal weight the sil will be more tear resistant, important on a light tarp imho. Abrasion i am unsure.
The rab sil tarps look good, been scoping one myself for 'UL' ground only use but not actually used/seen one. I have been using 3x3 to date, as said the range of pitches is excellent for a square tarp, but i mainly hammock camp.