This is a major issue for me recently, I have pretty bad chest dysphoria. I used to use the same solution you did, with trunks and a bikini top, but it's no good now it's too uncomfortable. I've not found anything that works yet, but since I am not large-chested, I'm considering trying a tankini top (not the ones with cups of course, just the plain flat fabric ones) or possibly trying to fashion something from the top half of a women's one piece swimming costume. No idea how well these ideas will work though!