The 400E and 500E battery compartment areas are quite different from one another, and I don't know that a 400E battery compartment is that easily adapted to a 500E. This may be a situation where something custom-fabricated along original 500E lines may be the better way to go.
@JC220 -- this could be an excellent line of business for you to get into -- having a metal fab shop create some new 500E specific replacement battery compartments. Not only for forum members, and a few units for your hoard, but also for the numerous 500Es that we all know are in your future as you slowly buy up all of them in the UK, as well as W140 and W220 models for your fleet.