Someone might be able to correct me here, but I've always been under the impressions that home cinema hardware, and amplifiers especially, don't take too well to being enclosed due to the lack of airflow for cooling purposes?
Enclosed isn't that good indeed, at least ensure sufficient air-flow by having an inch or two between the equipment (in particular the amp) to all sides and the top.
Nice and reasonable racks can be found at furniture markets (those huge malls I mean) but avoid IKEA crap.
The problem is that the specialist AV furniture manufacturers make stuff that the majority of AV nuts will buy - enclosed cabinets don't mix with expensive AV equipment & only people with expensive AV equipment will pay the prices they charge.
AFAIK the only quality AV Furniture manufacturers who make cabinets in glass are Stands Unique (but I believe that they only use wood for the supports) and Quadraspire (wooden sides).
Bear in mind that cheap AV supports in glass are not really feasible due to the high quality of the glass required - the Optimum stand I considered was capable of holding 300 KG - it really is a very specialist market.