PCPaul wrote:

Got a router or tablesaw? Or at a pinch (and if you dont value your
fingers) a circular saw?

If so, make a 20mm square U shape from any bits of wood and put a
groove down it using whatever tool does it best for you. A 'normal'
circular/ tablesaw blade will give you a 4mm slot, more if you set
the blade slightly off centre and rip it both ways round.

Thanks Paul - I could also make some channels by sandwiching 3 layers of
thin material, clamped & glued etc, or by routing a length of planed timber
to a thick L shape, but I'm still holding out for an off-the-shelf bodge
solution (or a graceful serendipity if you prefer).

Or, thinking creatively, let the bloody cat out. It'll come back when
it's hungry.

Yeah, we tried that - the vet's bills are eye-watering though. Damned
thing staggers home after 3-4 days with half it's fur left and limbs hanging
off, but then seems keen to go out and do it again as soon as the bandages
are off....