Here’s what I’m thinking. Drill some holes around the perimeter, put rebar in the holes, build your wall forms around it, pour cement, give the forms a thorough tapping to help release air pocktes.
You’ll need a masonry bit and hammer drill to do it. When it’s done, flip it and cover the rebar ends with more cement. You want them fully encapsulated.
I understand the tools might not easily available on a budget, but I don’t think the walls will make a very strong connection to the base if just poured over.