The space blanket only deals with radiation heat loss which is typically the smallest proportion, you need to deal mainly with conduction and convection. As soon as you lie on the ground then you and the ground will attempt to achieve thermal equilibrium and it’s a good bet that the several trillion, trillion tonnes of earth and rock will win out over the 100Kg or so of flesh. So having some form of mat to provide insulation is a good idea.
The shelters mentioned above can all provide protection but what they all lack is structure. It’s all well and good having something between you and the howling gale but if it’s pressing against you and the casualty then a few fractions of a millimetre of nylon aren’t going to do much good. Two bikes upside down and a tarp or similar pegged out across them will give you a better environment and also some room in which to deal with the casualty. The Rig3.5 mentioned above has pull-outs in the middle of the tarp so you can tension things better.
The Rig3.5 weighs 300g, add in a few pegs and line so call it 400g. An inflatable insulated sleeping mat is in the region of 400g – 500g. Both pack down to a litre to litre and a half in size. Worth considering in a group if you are heading out on exposed or remote trails.