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This is what I initially thought, but then couldn't work out if they used some sort of pull string.
The water supply hose is a hand tighten, plastic nut so you need to at least get your arm in there. Usually people don't like fucking around with what they can't see, so it's usually done so that the machine comes all the way out or at least out and to the side a bit. You'd likely struggle to find a too-short hose anyway. Caveat - there is no accounting for total shit-housery.
Marine ply might be overkill, but would survive occasional wetting better than chipboard. There are dedicated risers that might be even better.
This is what I initially thought, but then couldn't work out if they used some sort of pull string.
Not sure if it's clear from the photo, but there aren't any under the units / washers. Would a good solution to stop things getting stuck again be to put a square of ply the same thickness as the height difference?
Marine ply should be fine right?