I also microwave uncoated white fish, not using full power and checking after three minutes. I've got a couple of plastic bowls with vented lids, but if the fish is too big for the bowls I use an oval porcelain dish and cover with cling film that doesn't touch the food. I make a hole in the film and put a little patch of film on the hole, it will blow off when it fills with steam. I have tried the venting plastic bags but they're expensive and can't really be cleaned and re-used.
I find that the fish is perfectly cooked, tender and moist, ideal to eat with or without sauce.
I also use it if I'm going to make fish pie so that it doesn't take so long to bake the dish once I've added the potato.
I have tried with oily fish but prefer it grilled, though kippers are OK using the lower power.
There are loads of vented covers for microwaving on a plate and perhaps this would be a good solution for you if you could get one that fitted one of your plates?