One thing to consider with the Europa, of course, is not that the battery is mounted high, but that the shoulder is mounted low.
I agree that the height of the battery mounting is not ideal in terms of keeping the C of G low, though I would think that moving it to the front (spare tire area) is a less desirable location in terms of weight distribution and car handling. Though the battery may be mounted a bit high, the mass
is close to the centre of the car, which is best for quick and accurate 'turn-in' and lively handling. Mounting it ahead of the front wheels may induce understeer and generally retard the car's handling 'quickness'.
I've been giving some thought myself, to the battery's solid lump of mass and its effect on such a lightweight car. A couple months ago I was looking into lithium batteries as a replacement for the lead-acid monster. It is not inconceivable for a lithium battery to be 30 lbs lighter than the current battery. This would have a beneficial effect on C of G (even when mounted in its normal location) and is a significant overall weight reduction for the car. The lithium batteries are also much smaller, solving another pain-in-the-butt which is needing to remove the header tank, etc to get the current battery out of its hole.
I need to renew my investigation on this matter. One disadvantage is cost. Lituium batteries can run to $500. Performance particulars of lithium still need to be investigated to assess its suitability for a road car, but my initial findings are quite positive.
If anyone has any lithium battery experience or thoughts, I would love to hear it.
[edit:] I see Mr Pfreen has beaten me to it. This looks like a nice solution.