Foreword:
I haven't tested this myself yet as I'm fine with a native Segwit wallet in Electrum that I personally use and you should be very careful and test it before actually moving funds into such a wallet. Use Testnet Bitcoins to check thoroughly and if you're also able to recover such a wallet!


Have a look at: https://bitcoinelectrum.com/creating-a-p2sh-segwit-wallet-with-electrum/

I have taken a brief look at the code of the Python script linked in above tuturial and at a first glimpse it looks OK to me. The quality of seed entropy should be similar to what Electrum itself produces as the script uses Electrum functions for its inner doings.
Here's the Python script's code: https://github.com/AbdussamadA/electrum-xprv/blob/master/xprv.py

Important note:
You should create such extended master private keys only in an offline secure environment which you can later dispose easily. TAILS or booting a known secure and fresh Live Linux system into RAM-only is probably the best. TAILS is agnostic unless you save something in the persistent storage if you use that. A Live Linux system in RAM-only shouldn't store anything persistent on storage media. Make sure you've disabled all network connectivity.

You should know and understand what you're doing when dealing with such sensitive data!