I would put it in a breeder net that's partially above the water surface if you are just wanting to contain it and keep it from spreading across your whole tank.

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Go get a piece of fishing line off the old man's reel and 4 suction cups at the dollar store (they come in packs of 16 for a buck). Tie the measured length of fishing line that you need to make an invisible fence to a suction cup on each end and attach the cups, one to the front inside under the trim right at water level, and the other one to the back side under the trim right at water level. Repeat for the other side of the "corral" and your lettuce will have a nice invisible pen to live in. If you look really carefully, you'll see the line in my tank right here:

 

Clever idea, but you have to maintain the perfect water level for that to work, right?

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Nope. I have an open top tank there, as you can see. It will vary at least 1" up or down through the week as I top off the tank from evaporation. The water lettuce has looong roots so they stay right where I want them throughout the water level change. I tend to keep the fishing line just at or slightly higher than the water level when it's topped off because otherwise when my air pump kicks on to break up the waxy layer of bio film the water trapped in there won't get agitated enough to break it up. By slightly I mean 1/16" max, but I don't obsess over it.

 

Basic concept works with all sorts of variants. I've used airline hose, thin strips of acrylic cut from a light box cover, some random bit of flexy black plastic found at the bottom of my junk drawer, etc.

Right now my 37g is divided in two via a length of eheim hose cut to the width of the tank and slit lengthwise so it slips onto the mid-tank support. Clipped in place with a couple cable ties, it adds just enough bulk to keep the majority of the floaters in place on one side AND provide a break from excess surface flow so they don't all get blown up against the end of the tank.

 

I just added some to my 12g Long last night and the circulation has caused them to accumulate in the back right corner, and the front left corner, that is cool with me. I plan on removing them in another week or so once my tank is cycled.

 

I've used airline hose strung across the surface to hold floating plants like Riccia and Glossostigma to a corner. In the beginning you can see the hose but eventually the plant grows over it where you don't see it. The glossostigma will grow over the tank edge then it holds the whole mass by itself.