Hello,
I have created an online application for preparation of input files for generating SQS using the ATAT mcsqs. I am sharing it here in case someone will also find it useful. 
Access to the application here: https://atat-sqs.streamlit.app/
Or at GitHub: https://github.com/bracerino/atat-sqs-gui
I made also video how to use it: YouTube link
You can upload the initial structure (POSCAR, CIF, LMP, XYZ (with lattice)), or retrieve it from MP, AFLOW, COD databases, set the supercell (n. of atoms), elements and concentrations (automatically recalculated for the achievable concentrations for the set n. of atoms), set the parameters of ATAT mcsqs, generate the rndstr.in, sqscell.out with information how to run it in the console, or download a comprehensive bash script that will automatically create rndstr.in, sqscell.out, and run the set mcsqs settings. The bash script provides also more detailed information regarding the convergence of objective function.
The found best SQS (bestsqs.out) can be uploaded back and converted to POSCAR, CIF, LMP, XYZ format. You can calculate PRDF for the bestsqs.out, and create vacancies by deleting some element. Also you can upload mcsqs.log file to see the objective function over step, or upload multiple mcqs1.log, mcsqs2.log, … from the parallel run to automatically see the run with the best SQS (and other information). The last thing, you can upload a .csv file if the ATAT mcsqs was run with the bash script, and see the convergence of objective function over simulation time.
If you wish, you can provide feedback to [email protected].
Thank you