Are you referring to a spotlight? What do you mean by "sloped"? I looked on Grady's site at a 330 Express and it looks like the hardtop is slightly domed (slightly convex from port to stbd). You can make a adapter plate if you want. Or, just put a small stack of SS washers (or possibly nuts, depending on height of "gap") under the sides of the mounting plate then fill the gap with white sealant. Easy peasy and in the end it will look the same as a white starborad adpater plate, but much easier and cheaper.
Make sure the light is far enough back from the front edge or you will get lot's of glare on the foredeck.
On the other hand... you might just want to get a handheld spotlight. Less expensive and, quite honestly, more effective.