I’ve found the perfect enclosure for my Prusa i3 MK3 3D printer (no modification to the printer needed!). It’s a cheap server wall mount cabinet for only 79,- EUR.

You can find it on ebay or in different computer online shops. It has 4 mm safety glass and powder coated sheet metal walls.

The roof and bottom have cable entries and the roof is also prepared for mounting 2 x single fans. That may come in handy if you want to build a fume exhaust for ABS printing.

Check out my blogpost for more in-depth details with links etc.:
http://www.schweinert.com/perfect-3d-printer-enclosure-for-prusa-i3-mk3/

You can also add a recirculating air filter into your enclosure like the AirManagerV2: https://www.prusaprinters.org/prints/16996-airmanagerv2-recirculating-air-filter-for-3d-print

Update: I printed Nylon PA-CF with it in summer and the temperature maxed out at 44°C. Hot air can easily escape through the upper fan slots so I don’t need to worry about ventilation or cooling the enclosure.