Are you sure they are slugs? It looks more like caterpillars to me, and those you can catch. Check the leaves underneath a few times a day and remove them.

definitely slugs, They hang down near the hydroton and under the leaves....100% slugs all the way....

OK,
I suggest you get some slug poison (pellets) and put a few pellets on a piece of alum foil on the raft.
Even so, try and remove as many slugs as you can find (dead or alive) at least twice a day.

A traditional slug trap is to cut the top off a beer can and set it flush with the crawling surface. Leave some beer in the bottom.
The slugs will crawl in there and drown. At least that's how I've always read about it. Try googling 'beer slug trap'

With hydroponics as opposed to soil growing, it's not as simple to use many of the slug solutions. You don't want to add much weight to the raft if possible, and you want to make sure you're not adding stuff to the nutrient solution that will cause problems.

With your design I'd focus my efforts to the area around the tubs. If you can keep them from crawling up the sides you should be able to stop them cold.

You might also try ringing the upper rim with duct tape. Tape around the rim so that half the tape (long-ways) is sticking up with the sticky side pointing outward. So what the slug sees as it climbs up is the top of the rim, and then an immediate sticky wall going up a little further.

Then, coat the sticky part of the duct tape with as much salt as it will hold. Just don't let any salt into the reservoir because it's bad for plants.

Also, I'd recommend looking into the reproductive cycle of slugs. If they can reproduce in the conditions found on the raft, hydroton, or reservoir you may have a bigger problem on your hands.

Since lettuce doesn't need pollination, build a cage using something like the stuff they use for windows and screen doors, usually you can buy a pretty big roll of it.

Leave it to me to completely miss the simple solution. I'm trying to get all fancy with a salt version of a barbed-wire fence, when a fence version of a fence is all you need.

Nice call nicky.

I decided that the duct tape and beer solution was easiest, and so far so good, although the beer doesn't catch all of them. Thanks for all the suggstions, much appreciated - this is one of the reasons I prefer to grow indoors - minus the elec bill!

I hear ya man,

Solar growing is where it's at.

It's where the best growth is at.
It's where the best energy savings is at.
It's where the bugs are at...