I have these too Anti-Fatigue Foam Mat Set, 4 Pc. and paid full price ($9.99
) I tend to not interlock them but instead, just spread them around and move them as needed for kneeling / sitting

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I use a padded mat if I need to spend a bit of time to clean wheels, but I haven’t invested in a quality pad yet.
I also got one with wheels only to figure out that on any slanted driveway it becomes annoying.
where do you folks get them brush holders that hook to the side of the wash bucket?
Any issue with the foam cannon coming off the bracket? This too, i made my own version.
As soon as I spotted one of these, I just HAD to have one as it suits my wheel cleaning method so much. As usual living in Hooterville Australia, no one was selling one locally. I looked at importing one but $120 shipping on a $20 piece of bent metal seemed a touch excessive, even for me! I then looked into having one made but eventually a local supplier started to import them.
When I start to roll on to the next wheel, I generally grab the bucket where the holder is to steady it, however I don't do that every time I move and haven't had it fall off yet.