The Padfolio I bought was the perfect size for my 7" tablet. You should be able to find a larger one to fit larger tablets and iPads.
Cut two pieces of elastic, one for each side of the tablet. The pieces should be roughly as wide as the tablet -- don't cut them too short: they should be stretched to accept the tablet and hold it securely.
Apply a dab of glue on the bottom of the elastic band as well as on the case itself where the band will be applied. Let the glue dry for about five minutes then test fit the tablet.
The total cost of the project was $11.56 from Walmart; the most inexpensive comparable case I could find in stock at my Walmart was $29.98.