I've had good luck on smaller irregular sized parts by filling a container a couple inches deep with water, then placing an oversized piece of plastic over it and setting the rusted part in it. once in, start adding rust removal liquid and at the same time bring up the water level outside the liner until the part is submerged.
I think you could do something similar with your vat and a piece of pond liner and wood cribbing or plywood at bottom to insulate the liner from sharp edges. Using this process one can use far less rust removal product. Large voids in casting can be filled with other containers (i.e. milk jugs0 full of water. On small items I use marbles, paperweights and other rusted parts. Basically using water outside the liner and other items to displace rust removal product against the part.