i need help identifying a piece of equipment

a while ago, i was doing a boot camp session, and we used something for squats, i'd really like to get one for home but no idea what it actually was so here goes the description
it was approximately 2 foot long (ish), it was horse shoe shaped, appeared to be full of sand, and was made from some sort of rubber/plastic material, should add it was heavy but not really heavy
any ideas?
thanks in advance x

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