An inexpensive plastic box...

[Dannyalcatraz]

...perfectly suited for use as a gelatinous cube!

Small Ball Display Cubes: Golf Ball Display Cube Item #44010

They come in various sizes, the smallest of which is a 2" cube used for displaying golf balls. Sounds perfect to me!

And they're only $1.99!

If the flawless, hard plastic clarity of them annoys you, you can always get all crafty on it and use hot glue or urethane or some such to give it a more organic texture.

 

[Cor_Malek]

I think I recall someone writing a piece on cheap home-made dungeon-tile-ish setup, and recommended those to emulate flying/hovering stuff.

Probably Weem ;-)

PS.: Yup, I remembered right: Free/Cheap “Dungeon Tile” Ideas | d&d done wrong | d&d thoughts and parody
He even found some for $1.29 and $0.99

 

[Dannyalcatraz]

Those are pretty cool, but those are rectangular.

These are...CUBICULAR!

Oh yeah, while we're at it, a better choice for the flying critters would be these plastic risers for displays: with only 3 sides, you can put stuff under them with ease.

 

[cattoy]

I just use the plastic box that my d6s came in. It covers a 10x10 space and you can see through it.