OK - I'll have a go, but you might start to feel less technically challenged when I have
The poly box is one of the ones that they deliver organic meat in, complete with lid. We bought one length of the small bore drainpipe (about 35-50mm -ish diameter) and the thing that connects it to the gutter - don't know what that's called
and various connectors - right angles and a couple of T-pieces. The box was sunk into a big hole in the ground, level with the top of the lid and then filled round to give a tight fit. We stood the gutter thing upside down in the box to lift the pipe up off the bottom and connected the pipe to it, then filled the box full to the top with gravel and put the lid on it - you need 2 holes in the lid - for the in and out pipes.
The top of the pipe had a right angle connector fitted and this went into the back of a small wooden box (which we made out of some scrap plywood) that housed the fan. It's not a big fan - came complete with solar panel from fleabay (about £13) can't remember the power (?) of the panel. The box was wedged into the crop bar/polytunnel frame joint.
Then we put a right angle connector into the outflow hole in the box lid and joined several short pieces of the drainpipe to the t-pieces to give various outlet points along its length. This runs along the ground next to one of the raised beds.Then we covered the whole path, including the top of the box, with weed membrane and clinker from the boiler - because we had tons of it and it makes quite good paths.
The only problem we've had is the top of the box is a little bit spongy although it's safe enough, being full of gravel. And the fan box isn't very secure - we've not found a reliable way of attaching it to the frame yet. But that's a minor problem - it doesn't fall down just doesn't stay wedged in IYSWIM.
OH has just reminded me that we cut up a pair of my old tights and covered over the outlet holes to stop soil/ clinker etc getting in. And we siliconed all the joints on the fan box to make it as airtight as possible. And make sure you put the fan the right way round - so it's sucking the air into the pipe not blowing it out!