A proper Milford cargo barrier will be safer than a net, especially if you're stacked higher than the back of the seats.
I had one in my 80 Series and it was fixed with two large wing-nuts to the roof brackets and two bolts to the floor. It took maybe 20 minutes to remove. They do require the fitter to drill some holes for the brackets.
Good luck getting one fitted in a week.
When you think about all the science involved with car safety, and the amount of kinetic energy possessed by a 100km/hr Engel hitting the back of your head, I'd be wary that a large piece of MDF wedged in the back doesn't increase the risk of injury rather than prevent it. You did say you wanted to feel safer.
Ek= 1/2mv2 (mass in kg and velocity in m/s)
What's a fridge weigh? Say 20kg fridge moving at 100km/hr (ie about 30m/s) is 0.5 x 20 x 900 = 9kJ
A 5kg suitcase would be a quarter of that. Still gunna hurt.