wrap vinyl, carpet or whatever around a bit of mdf/wood that you make up to form the 'L' thats left behind.
thats what i did for my mini UHF i had in my lux, and then i found the an ebay spec surround and put that it, 40 chann then just slid straight in.

i guess you could even make a full surround, and wrap it?
idk, ive got my turbo timer on the RHS of my steering coloum, also have my keypad next to it, both kinda just above the i/c spray button

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my05 STi
my13 Rok

:)