I'm looking for an easy to put on and easy to take off sealing cap for a five gallon bucket so that I can check the progress of my rooting cuttings. I thought if a shower cap will stretch that large it should do the trick. It is waterproof so it will containg the moisture and be easy to put on/off. Any one know?

Home depot carries easy off lids for 5 gallon buckets, don't know if a shower cap can fit.

How about rummaging at your local glass shop for squares of tempered glass? You could use plexiglass too. I think Home Depot cuts them to size.
There used to be a great store in my area, Habitat for Humanity Restore, that sold all kinds of donated building materials. I picked up stacks of glass for projects for next to nothing.
The tempered glass doesn't break as easily, and usually has a nice beveled edge that won't cut you. If you go with regular glass, you can duct tape the (sharp) edges to prevent a visit to the E.R.

I've also seen bowl covers in the market and dollar store that have elastic to hold on the rim of a large mixing bowls.  Like shower caps only for larger openings.  Good luck!

have you thought of saran-wrap? then cover with aluminum foil. let the cuttings sweat. open the lead every 3-4 day to aerate them for 5 min the seal  it again, no light.

You can get drum covers for 30 and 55 gallon drums, which are plastic with an elastic strip, that look and work much like a hair net or shower cap. They work great.

And there you go

http://www.interplas.com/5-gallon-pail-cover-dust-caps-4-mil-p-dlc-05 

$98.50 for 100 covers. Not sure I'll go that route but the answer is clear.

Thanks Jon!!!

what Orange thing do you have in your wallet  Charles? Sounds like a magical wish card. LOL

This year I used what I had at hand, so a dark plastic trashcan bag. I would seal it with the weight of the bucket itself - so the opening of the bag to the ground.
The advantage is they use less space of storage, and you can use them as trashcan bags and buy new ones next year - so don't have those things (whatever the covers you choose) lying around all year, and
being broken while you store them on the side.

I think these are what you're looking for.  They're large and should work for 5 gal buckets.

http://www.amazon.com/REUSABLE-ELASTIC-FOOD-COVERS-ROASTERS/dp/B0053HDD6S/ref=pd_sim_k_5

Good luck!

EDIT:   WHOOPS! I just read the reviews of that product and it looks like you should be cautious.  Some buyers report that they got smaller covers than advertised.  Sorry!

This looks like the right thing at the right price..

http://www.materialstoragedirect.com/cdf-elasticized-pail-dust-cap-antistatic.html?utm_source=google&utm_medium=shopping&gclid=CP_KxPTcgrwCFWMV7AodqBoADg

if that shower cap was made for my head, i'm sure it will fit 5 gal bucket x)