# [MATL], 19 bytes

    tZ"ibpyn&Z@(TT35&Ya

[Try it online!][TIO-k1as67xa]

[MATL]: https://github.com/lmendo/MATL
[TIO-k1as67xa]: https://tio.run/##y00syfn/vyRKKTOpoDJPLcpBIyTE2FQtMvH//2gTBdNYLvXEpOSUVHUA "MATL – Try It Online"

Thanks @LuisMendo for saving 3 byes, so now it has the same bytecount as @flawr's [answer](https://codegolf.stackexchange.com/a/193908/32352), but sufficiently different to post anyway. High-level agorithm overview:

     Z"                  % Create n x m matrix of spaces
               (         % Index into this matrix:
       i                 %  The alphabet vermicelli (explicit input)
            &Z@          %  at a random locations (randperm), which are
          yn             %   length(S) numbers, ranging
    t   bp               %   from 1 to n*m
                TT35&Ya  % And finally add a border