You could line it with landscaping fabric (weed membrane)if soil leakage is what you are bothered about, or you could just remove some ot the pebbles around the perimeter. Once the soil is settled though there shouldn't be much leakage.
My raised beds sit directly on the soil,allowing worms and other microbiome inhabitants access to the bed. It also allows deep rooted plants to reach nutrients from the soil below.
I am trying to go down the 'no dig' route, though I did clear the ground first as there were a lot of deep rooted weeds like dock and willowherb that I didn't want to persist. I used mine to help terrace a slope which meant I could fill them with the surplus soil and then added a thick layer of well rotted manure. I have already, in the first year, noticed the difference in soil drainage and workability. Plants have grown well and weeding is much easier. than before.