If your van has an internal pump and your pick-up pipe has just a removeable (unscrew) filter on the end, get a length of white water pipe, remove the filter, stick the small pipe down the large pipe and replace the filter. The large pipe needs to have a big enough ID that the screw assembly on the end of the flexible pipe will fit down it, and it needs to be long enough that it holds the filter at the bottom of the Aquaroll and fits just under the lip at the top of the barrel.
Despite what it shows in the catalogue, you can buy an individual 2m length of pipe from Toolstation.
If the pump is external its own weight should hold it at the bottom.