I've made a plant corral in my QT tank using the plastic edging that you put over corners when you're doig drywall. It's a couple of bucks for an 8 foot long piece, it looks like this:
I cut a piece the width of the aquarium and cut the "spine" out of either end to create a notch that fits over the sill that the hood sits on:
And put it on like this:
From the front it isn't very noticeable:
Here it is from underneath:
It doesn't work with all aquariums, and the water has to be close to the sill, but it's cheap and easy to use.
I've also used it to create a baffle on an HOB filter by attaching it to the outflow area using nylon bolts.