Thank you all so much for your replies. I think I'll take Swingman's
and George Cawthon's advice and skip the particleboard substrate.
Since the desk is in my own house, I can always go back later and
stiffen if should it not work out.

And per George's "Real problem will be keeping your pen from digging in
to a plywood top. Use
a blotter. "

Absolutely.

Thanks again everyone - your input is very much appreciated.