We started potty training Roman in December. It's going well, but we still need something on his bed at night (we don't use Pull Ups & I've yet to buy/make training pants so he just wears regular underwear). This idea popped into my head so I went w/ it. :)
Take a flannel receiving blanket
(I hate measuring things out so, when I don't have to, I don't!)

Put your waterproof material (I used PUL) underneath & cut around.
Or do a better job than me & measure/trace. :)

Then get your absorbant material (I used Zorb) & lay your blanket on top of that. Cut around.

Sandwhich together your layers: PUL (shiny side in), Zorb & then blanket.
Pin.
Serge.
Et voila!

What we do:
Put a sheet on Roman's bed.
Then the pad on top, in the area that would most likely get wet.
Put another sheet on that & make the bed like normal.
Then, if he's wet in the middle of the night, I can just whip the wet sheet off & not worry about re-making the bed.