They need roughly at LEAST an inch of room in all directions around the amp (except the mounting surface). A few fans will also be a requirement. Good air flow is needed to keep the amp at same operating temperatures, and the cooler it stays the longer the amp can last. Heat rises, and with a plexiglass top over the amps, the heat wont be able to rise, so the fans will push cool air over the heatsinks. You may also want to think about mounting the amps to a piece of thick aluminum and using heat sink compound between the mounting points and the aluminum. This will help draw heat away from the heatsink of the amp.
Amps do get very hot, even if you're not running them at their full output capability. Especially if you're running them bridged or at less that 4 ohms.