Cutouts in worktops or countertops have exposed edges by their very nature. Depending on the purpose of the cut-out those edges may be visible when the project is complete (e.g. around an under-mounted sink) or hidden (e.g. tap holes and hobs).
Where the exposed edges will be seen the finish has to be good, which means the material you use for the cut-out has to be smooth and free of defects. Where the edges are hidden less care has to be taken over the finish but the shape still needs to be accurate.
Another consideration regarding the material is how easily it will de-bond or demould from the concrete. Anything that is rigid may cause corners to chip when demoulding takes place.
The absorbancy of material is also another really important consideration. Concrete when curing relies on moisture being present. Any material in contact with the concrete that sucks the moisture away will make the concrete weaker at that junction. In the case of an exposed edge, that edge will be rough and in fact bond with the concrete.
For cutouts where the edges remain seen, the simplest option is to use the same material and system as you would for the outer edges – melamine. Foam can be used but the edges should be treated or taped to prevent the foam from sucking up too much moisture from the concrete.
For cutouts where the edges are not seen then foam alone can be used.
MDF can be used but should be used with caution. Without being treated to make it impervious it will leave a rough and unsightly finish. It may also chip the concrete when removed as it will bond to the concrete surface.