Axminster have a 'Deep Throat G Clamp'. They are described as being light but I imagine they are steel, not aluminium. There are two sizes 50mm x 90mm at £5.20. and 75mm x 120mm at £7.99.
I haven't tried these but I do have their One Handed Clamps
which I find very useful and well made, like all their tools. Axminster is my first port of call when looking for tools and as I pass their shop on my way home from the lake, I frequently call in just to buy some Titebond
or even have a look around.