Even Apple has not gotten around this. The antenna gate phone achieved this by building metal as part of the antenna system - which did not work terribly well and they had a massive team of the best RF engineers in the industry to figure that out. The newer iPhones have glass on the top and bottom where the antennas live.
Even on things like the iPad and Mac, the plastic Apple icon covers the antennas. If you look at most laptops, you’ll notice even if they use a lot of metal there is usually a plastic strip on top of the display where the antennas reside.
Your best bet is to maximize the amount of plastic near the antenna. You can spend a LOT of money in the lab testing different variations to see what does and does not affect the antennas, and what changes can be made to improve performance but that depends on how much budget you have.