Although Einstein entertained relational ideas in the early years under Mach's influence, he did not support a relational spacetime after developing general relativity. He believed that it requires spacetime to be a physical entity, albeit different from Newton's, and was even willing to call it "gravitational ether", see his address Ether and Relativity (1920). It was delivered well after his hole argument of 1916, which is considered a strong argument for relational spacetime, meaning that Einstein was not impressed by it.
Einstein identifies the physical nature of spacetime with the measurement relations expressed by the metric tensor and the ubiquitous gravitational field. However, it is a peculiar entity that lacks its own mechanical properties, and only manifests physically by setting up measurement standards and affecting ponderable bodies and other fields. So it is, perhaps, unsuitable to think of it as a "fabric" made of something, which we'd expect to have such properties of its own.
Considering that the invariant essence of a metric is its curvature, one can also say that Einstein's spacetime rather manifests by "curving" other fabrics, like mechanical and electromagnetic processes. Some authors even go as far as to say that it is "made of geometry", being an evolving Riemannian manifold. Here is Einstein in his own words:
"This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials g_mn), has, I think, finally disposed of the view that space is physically empty. But therewith the conception of the ether has again acquired an intelligible content although this content differs widely from that of the ether of the mechanical undulatory theory of light. The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events.
[...] As to the part which the new ether is to play in the physics of the future we are not yet clear. We know that it determines the metrical relations in the space-time continuum, e.g. the configurative possibilities of solid bodies as well as the gravitational fields... There can be no space nor any part of space without gravitational potentials; for these confer upon space its metrical qualities, without which it cannot be imagined at all. The existence of the gravitational field is inseparably bound up with the existence of space. On the other hand a part of space may very well be imagined without an electromagnetic field...
[...] Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."
