sandwich package is great for correcting heteroskedesticity and autocorrelation, as well as for clustering standard error.
There are several instructions online talking about how to cluster standard errors in lm and plm setups. All of them mentioned that the degree of freedom needs to be corrected.
However, in the newest sandwich version: 2.40, the new fucntion, vcovCL, includes the option of degree correction.
Here is the change log of version 2.40
Various flavors of clustered sandwich estimators in vcovCL(), panel sandwich estimators in vcovPL(), and panel-corrected estimators a la Beck & Katz in vcovPC(). The new vignette("sandwich-CL", package = "sandwich") introduces all functions and illustrates their use and properties.
The new function vcovBS() provides a basic (clustered) bootstrap covariance matrix estimate, using case-based resampling.