The TopoLite Indoor Grow Tent is affordable, comes in many different sizes, and includes a viewing window.
The viewing window makes it easy to check on your fruiting mushrooms without even unzipping the door.
The mylar interior reflects light, allowing you to use minimal lighting (especially if you use open shelving, which will allow enough light to penetrate your lower shelves).
The mylar bottom includes a removable tray, which makes it easy to deep clean your fruiting room between major flush cycles.
This tent is airtight and leakproof, but if you allow enough water to pool up on the floor, you might see some water leaking out. But of course, you would never allow standing water to endanger your precious mushrooms with the chance of mold, anyway — you keep your shop vac handy near your grow room for regular clean-ups.