Each of the steel cabinets is surrounded by panels that will take most of the weight of the top platform.
Then I screwed a 'rail' to the outer wall to support the panels at the edges. My table saw is working overtime on this job! But it's nice to get all the joints clean and snug ...
And here we are ... there are nine pieces making up the platform; it's 25mm ply, and seems strong enough to span those openings without flexing much.
Some of the pieces will not be screwed down, but will sit 'loose' in place. The ones in the centre of the platform will open up so that I have the option of sitting with my legs down in the opening ...
... and the ones in the back corners lift out so that the spaces beneath can be used as storage bins. I'll try and find some flush-type handles for these next time I'm at the hardware store ...