Here's a Rube Goldberg; cover the entire bottom of the amp in velcro, cover that with cellophane and slide it in there, then pull the cellophane out. Use industrial strength velcro. There is enough carpeted flat space to get a good hold, theoretically, in my pea brain.
That's what I did to my amp in the the rear cubby hole and to an amp in the S2K. No problems so far.
08 X Sport/Ent -- Sterling Gray/Ebony
03 X Touring/RES -- Taffeta White/Saddle
02 Honda S2000 -- Berlina Black/Black
99 Acura RL -- Naples Gold Metallic/Parchment
12 Toyota Avalon Limited
12 Toyota Highlander V6