Does anybody know if such a thing exists as an attractive wooden shallow box to put growbags in? I'm rather sick of the garish growbags and want to disguise them somehow...

Don't know about boxes, but one year I modified some hessian sacks to make rather attractive rustic-looking covers!

My other half has made tannelised (spelling?) timber holders for growbags - two deep of course, throughout my two tunnel tents.
We use a polythene slip in the bottom of each holder 'cos that makes removal of bags easy (as if you were putting a foil slip under a pudding bowl being steamed, if you see what I mean). I put the first bag on top of the slip, cut out two thirds off the top surface. Place the next bag ontop. Cut through, depending on what I want to grow - either two or three planting holes.
They work brilliantly and look tidy.

that sounds interesting. So I take from that that you don't think a single growing bag is sufficient?

Hello there
Not sure about wooden ons but I have several made from re cyled plastic they look quite bright but do hide the bag I bought them at my local garden centre

If you use double bags it reduces the amount of watering, gives top heavy stuff - like tomatoes and cucumbers - more stability and allows closer planting of the lightweights - peppers, chillies, melons etc. The yield I got from this method last year certainly justified the additional cost and I only had to water once a day for most of the summer.