I want to use magnets on my base bottoms and magnets in my tray for a really good grip!

Don't do it!

You will have a great grip in one direction (think north-south polar alignment), and horrible grip in any other direction.

Use a combination of flex magnet and flex steel to avoid polar alignment issues and simply use Heavy Duty Base Bottoms or Sheet Magnet for better grip.

Most of the grip from sheet magnets comes from the sheeting being in intimate contact. Keep your base bottoms and carry trays clean.