Greta an average single chair seat is about 19" in width & a double seat is about 32". This could make 24?? cosy & interesting, depending how big the warm props aft ends are! Also I assume the height being of a normal chair?
If so you can use a sturdy wooden crate, that has a secure top. Fix & fasten to the base board securely, to give adequate support. Then cut polystyrene pieces to shape around the crate. You can either use calico/muslin to hold foam in  place or pour on polyurethane (or simular) insulation foam, to give the required stalagmite texture effect. Then painted in keeping with the mise en scene. Depending on it?s height?  It may only be awkward rather than heavy!
Check out your local Insulation Company?s or hardware suppliers. As there a myriad of simular pressure pack expanding foam products available. However while this may not be cheaper than the other method. The foam expands dramatically  in air to give a large coverage.
Hope this helps?