1) Picture it = check it out?
2) Solid gold = solid as gold?

David Copperfield said when he was to show his flying skill.
See, I'm in this big Plexiglass box.
Picture it so solid gold on the back, you can examine the backside.
'Picture it' usually means 'imagine it', Jobb, but this narrative you have posted is not very coherent.

'Solid gold' means 100% (24 karat) gold.
What confused me is that the box is made of transparent glass, not gold at all.