I somehow have trouble believing those colors are what it looks like! That is a very bright fuchsia color. That being said, it would appear this is a high quality of stone with lots of broad flash fire and, if I understand your post, you plan on capping it with that small crystal cap, thus cutting off half the stone and much of the best fire?! Maybe I'm missing something here, but I would never do that. How thick is the opal? Is it very fragile or thin that it needs to be a doublet? If it needs to be a doublet, personally, I would leave the front natural and back it with optical quartz entirely (if the back is not worth showing, you could back it with opal matrix, basalt or black jade) Or, if you think it needs a quartz cap to make it hardier for rougher usage, I'd cover the whole stone with a larger piece of optical quartz and finish it in roughly the shape it is already in. But perhaps I am misunderstanding your intent or there is some piece of information I am misinterpreting here.
I have a largish opal right now that has great color but is a weird(er) shape and only 1.5mm to 2mm thick in most places. I could make it into a few smallish, thinnish solid opals, but am planning instead to back it with black jade. However, I too have been thinking and thinking about it, so I know it isn't easy to decide what to do.