Here is an interesting solution from one owner. He basically bought a spare engine cover and bolted a rack to it. This avoids the need to make any holes in your clam and means you can just swap the engine covers over, should you need to use the rack.

The Boot Bag is a cheaper alternative to a full rear rack.

Unique Rack is a new option from the USA. Designed from scratch this rack fixes through the bodywork to the chassis and sub-frames underneath the clamshells and provides a very sturdy rack. It's not cheap but it looks like a quality product and design.

Lotus used to have a luggage rack in their Autumn/Winter collection catalogue for £575. Most of the dealers that I've spoken to are not aware that it exists and from what I've heard from other owners, half of Lotus don't know about it either. Most of the 3rd parties that used to make these things no longer exist or sell them now. I believe Lotus now sell a bike rack for the Elise.

Frederic Koninckx Motors, a Belgian company manufactures a chrome luggage rack which is sold by the following companies in the UK (picture). A Microsoft word document describes the rack with contact details and prices.

Moto-Build Racing may still sell a rack. It include Elise straps and suction pads so no drilling is required.