I haven't come across concrete gravel boards yet.
One of the bigger problems is the way slugs are harboured in whatever I
try to put down. We may try nematodes (sp?) in the spring, but I
thought these were expensive when looking through the internet for
suppliers
I would be interested in your opinion on these

Tried nematodes one year on our potatoes and decided it was not worth the
cost, we could see no difference to previous years. Cost was about £25 from
memory and you can buy a lot of organic spuds for that.

--
Regards
Bob H
17mls W. of London.UK

That was about the cost that we sourced.
Did you find a useful alternative?
At the moment I'm reduced to plucking the slugs out as I spot them! i
also use slug killers but am hoping to prefer more organic methods

LW