The way I would do it is a wood frame built from 2x2's build it the way you want it to look... then put chicken wire all over it and SHAPE it with the chicken wire. From there glue foam onto the chicken wire big chunks of foam... then carve it.
In the end you either can spray it with a gunite type of spray which would then make it feel like real stone or you can hard coat it... depending on what you want the final texture to be determines what you will or won't use. Clearly in the end you light and paint the tunnel. There are other ways to do it like make molds and pull them from resin or fiberglass or whatever but for us the simple haunt owners we have to do it the cheapest. I think the worst way would be vac form it won't look great but it would look okay.