Using laminated furniture board as a plinth help please

A couple of weeks ago a plumber and his mate removed a downstairs toilet for us.
In it's place they plumbed in a washing machine which sits above the soil pipe.
To keep the job simple they made a plinth above the soil pipe which is secured onto batons of 4x2. The 4x2 is screwed into three walls. (Its a very small room just big enough to fit the washing machine and a hand basin).

When we use the machine it "walks" slowly across the plinth towards the edge.

Is there a simple fix to this?
My idea is to screw some beading onto the plinth creating a raised lip to keep the feet in place as they don't "bounce" on it just vibrate enough for the machine to move.

Thanks for looking,
Bern

How about a couple of the rubber cups you put under chairs screwed down to take the front feet?

Footprints,

Now that's a nice simple method.
I'll give it a go.

Thank you,
Bern

Yup or any kind of sheet rubber or upside down lino would do, anything grippy if you can't get cups large enuf

The other thing is check the feet, sometimes they get dust/crap stuck to them and the rubber becomes slippy, a clean up may be all they need. That and leveled properly.