I then moved them to a pen that was made of plywood on three sides with a glass front with a floor space of four foot by eight foot. The pen had several heat sources and a large hide box. The tortoises spent a lot of time exploring this cage but spent very little time banging against the glass. I think it is important to remember that glass is not part of their natural environment and most tortoises try to walk right through glass as if it wasn't there. I feel that a large pen with reduced glass may be a factor in reducing stress in the animals in some cases.
Ventilation is another factor that is related to the animal's native habitat.
Make certain that the ventilation allows you to maintain the proper humidity. Inadequate ventilation and a large water bowl may lead to humidity related health problems for arid species, just as an open topped cage with several spot lights may dry the air out too much for some species from more humid environments. Substrate is a topic that has been much discussed and very little consensus has been reached. Newspapers are the old standby and will work. They are cheap and readily available.