I wonder if this will work?
A removable aluminum hardtop for ST D90s would basically have 4 parts: the roof, 2 sides and a hatch. The roof would be the the most complex part to fabricate.
What if someone made a wooden auto body buck like those used by traditional coach builders to duplicate the roof? Then, this wooden buck could be taken to a specialist in car restoration that deals with aluminum bodywork to hand hammer a roof. According to Hemmings Auto News, such restoration specialists charge around $65/hour or more. So, $65 hour x ??? hours of work.
Maybe a couple forum members can get together by region and build the wooden buck together, or someone could build one and then rent it out to others, etc.
Just an idea.