No problem, I have cable tied 4 bathroom pvc vents together to create the initial square. cost £11
I purchased 3 6mm pvc sheets, 2x 320mm2 and 1 at 360mm2, they are bolted together with 45mm machine screws and 4mm bolts, cost £25
The lower sheet sits on top of the vents and has a 120mm diameter circle cut out the middle which the 120mm pc fan sits on, fan cost £4.50
Bolted to the fan frame is a 110mm diameter pvc cylinder which houses the sensor, cost £5
Today i purchased a circular vented/meshed cover for the bottom of the cylinder to protect the sensor from any debris as the fan has quite a good pull, cost £5
Total cost with fixtures is around £ 50, Thanks Colin