The A500 has a classic beauty and should not have modern proucts on it's left side. Period.
The free space that exists inside the case is vast if you compare this to a A1200 and a moden solution should be internal.
I had a viper520CD on one of my A500 and couldn't care less for any GVP or A590 side car.
One thing you should reconsider, if it is not to much trouble for you, is the memory trapdoor. On your ACA500 MK1 design, it ignored whatever existed there.
It would be really great if a chip ram expansion lives there to be fully useable as a...chip ram expansion