Hi VAnMan2014,
You may have read this already but there are some blueprints for DIY foundations and bedframes in the foundation post here that may be useful.
If the wood you are using is sanded or smooth and there isn’t a risk of wood splinters or sharp edges catching the mattress then you won’t need a fabric covering but if you do choose to use one then it can be anything from a box spring encasement (such as one of these) to a more costly fabric (cotton makes a good “default” choice) that you wish to use for aesthetic reasons.
Phoenix