The opening has a max height of 40.5 mm at midpoint and 80.4 mm max horizontal at the midpoint
The corners are rounded.
I have an analog factory clock now, but I remember on an 86.5 that I had I swapped the LCD for a factory analog from Devek and it fit perfectly.

some pictures of the LCD digital clock:

Fitting a UK Ford analog clock in place of the LCD clock

A guy in the UK did this, I just copied his idea, I only saw the picture he posted of the finished job not how he did it, and so the install details are mine.

The clock is from the 80's UK Fords, and it was fitted into the Capri Cortina and Granada’s, maybe others, there’s usually a few on eBay at cheap prices, make sure they’re complete with the bulb holders. My lcd clock had the black death but I prefer an analog clock anyway.

Once you’ve got the clock out you will see that the clock module is fixed to the console trim by the switches for the rear a/c. I made a aluminum plate (Not prettist thing I've ever made) to hold the clock and the a/c switches, the plate was a tight fit on the clock and glued in with JBweld, the clock needs to be held securely because on this clock to set the time you push in and turn the centre button.

I used the electrical connections from the old clock apart from the live feed which on my car was only 3v and wouldn’t drive the clock, I don't know why I was getting 3 volts, in the end I took a feed off the radio.

Hope this gives you some ideas, there maybe other clocks that will fit.