Yep, sand mix, sometimes people use mortar mix. The key is not to use concrete which has aggregate stone in it.
When I did this I used an acrylic pan but the connection was made to the drain with a rubber gasket after the pan was set. It just needs to be centered over the drain. Then, once the mortar has set, you force the gasket in between the pan and the drain pipe. Its a very tight fit. Never leaked.
many manufactures will tell you that the mortar is not necessary. That may be true if you dont mind some flex. The mortar will add a much more solid feel under foot.
When I did a drop in tub like this I built a dam out of cardboard to contain my lake of mortar mix. That statement made it sound like a wet mix, it wasn't, but it was woried it would slump out of place, the cardboard walls kept it where I wanted it.