I thought I would have a go at building a liquid gauge into a B5 vent. The first storke of luck is that the vent inside size is the same width as the liquid PCB, the rest was childsplay. I went down to my local scrap yard yesterday and was able to buy 9 vents at £1 each, result. First, remove the movable vents from the back of the vent. Once removed it will look like this. Next use a hack saw to cut out the middle vents. After that sand the plastic flat. Next a quick spray of black satin to tidy it up. Next cut a fascia out of 1.5mm aly and paint back, (I will get some of these made up) Drop in the PCB and secure with a couple of dabs of glue. Then finally drop in the fascia. Job done.