I like to use wrapping paper.
The good kind that has somewhat of a coating, and not just real flimsy
- thin like tissue paper stuff.
I take a piece of poster board, cut measure, tape together if needed.
Then I wrap it with the wrapping paper of choice (usually an ocean
blue, or holiday themed too).
Tape it to the back of the tank using packing tape, as that holds up
so much better than invisible tape.
It's cheap and easy.
You can easily change it to do a theme to match any holiday.
The type that has an obvious coating that make it have a mirror type
affect when hung up gives male bettas a "flare -buddy". That is until
they get bored.
The normal size of a roll fits on larger tanks. On my 75g the only
thing I had to piece together was the poster board.
You can use the cheaper, thin stuff too. However, when that gets wet
(and it will...only a matter of time) it will either dry wavy or even
show the water spots.
That's good to use when doing a theme like for the Christmas, and
it'll be only used for a short time. For regular use, get the good
stuff.
Silver metallic (IMO) doesn't work so well. My daughter wanted that on
her 3g once, and it just looked bad. Her betta blanched his color out
constantly.
I tried to tell her that even HE didn't like it, lol.
As soon as I changed it to a lovely metallic blue, he was a happy
camper. Strutting around like a rooster.