Charlie's finished trucks are spectacular and appreciate his sharing his skills, I believe that the Ford cab is from 1962 as I believe that was the last Ford pick up model until AMT's release of the Ford pick up in the mid 70's. I had the privilege of obtaining one of the castings and ended making a newer 1970 model where I had to make my own grill. Had a little problem with an interior but ended up with this:
If I understand this the raised part is going to be a recess with a license plate inside. then lower the whole platform you have it on and drill holes in the top plate at the corners to draw the vacuum. the license plate will cover them up. Or... reverse your mold and make the draw into the recess by little holes in the corners of the recess that would be able to draw the vacuum and pull the plastic into it greg 1 rivet, 2 rivet, 3 rivet, 4 rivet, oh darn lost count....
It appears in the photos that you are trying to vacuum form a negative form, that being the top is wider than the bottom, If that is true you have hard task to complete. To get the upper edge of the plate you might put some small holes in the corners that allow the vacuum pull towards the edge. If it has to be clear then make another form that fits around the upper form and push it on the plastic after the pull but still in the pliable state, my experience is with the Mattel toy greg 'I don't build with rivets'
I had one a long long time ago, It was a fun kit to build and matched up the visible engine to it, It all worked in spurts. One good thing that Renwal did was provide a replacement parts sheet where you could send away for new parts, My brother and I souped it up with lower grit number sandpaper on the clutch plates, a little more battery kit to the motor. Used vasoline to keep things moving as directed in the directions)
Have a few parts left, not many, It was big for a kid to move around. Had the radial airplane engine also. Parents thought good investment for a mechanically inclined child.