The only way to do it would be to build a new hood yourself that could accomaodate the metal halide within it obviously this would need to be a very deep hood and have plenty of ventilation perhaps with a £10 clip on desk fan from homebase included.
You could go for a 250 watt lowbay unit then as it would not be visibles its appearance would no be a problem.
I think the weblink is something like www.marine-lighting.co.uk
As far as making a hood to go around the curve at the front of the tank you can get bendy mdf panels that when staind will look like wood. Its a case of carfully designing the structure of the hood so the bendy mdf panel will be secured at at least 5 equally spaced points to maintain the required curve.