Rear Seat Delete Ideas?

I made a rear seat delete package tray for my fastback. Although I can't help yu with dimensions perhaps a couple pics could help you out with construction ideas.
I didn't want to use the flimsy fibreglass repro package trays so I made it out of sheet aluminum.

I made it in three sections for ease of installation. Also, I attached the 1st section (front) using a piano hinge so it can be raised. This enables me to use the space where peoples butts used to go as storage space rather than wasted space.

The tray is quite sturdy and there's no flopping or rattling at speed. You can buy the sheet aluminum in various gauges to ensure it's sturdy enough to mount a baby seat. I would think you'll need to make some sort of very solid sub-frame if you are going to trust a baby's well-being on it's construction. Also, I'd make sure the anchor belts of the seat pass through the tray and mount to the sheet metal floor of the car properly.
Dave