Had my boiler has recently been relocated from the utility room, and I
am now left with a 6cm x 25cm cut out in the worktop where the pipes
used to pass through.

It is a standard chipboard laminated worktop, L-shaped (mitred). As it
is in an almost invisible position, and will probably have a microwave
sitting on top on the cut out, I am quite happy to keep it, as long as I
can fill the hole somehow. The cut out is against the wall, and I will
tile to it once the hole is filled.

I am looking for a hard, preferably light in colour, water-resistant
surface.

Best I could come up with is to screw and glue a scrap piece of wood to
cover the hole, leaving a few millimetres gap at the top, and them fill
with wood filler to the surface of the worktop. If possible varnish the
top of the filler.

Will it work? Any better ideas?

TIA.