DIY Blind Filter from Single Baskets

I have recently purchased a Lelit espresso machine that doesn’t use the more or less standard 58mm portafilter, but rather 57mm. Since I need a blind filter for backflushing and cleaning the brew head I was wondering if I could somehow clog up one of the single baskets that came with the machine and use those. I only use the large "normal" basket anyways. I used a piece of plastic to try it with only water and it seemed to work but I’m reluctant to use plastics in combination with hot cleaning solutions.

What could I put inside the single basket which doesn’t react with cleaning agents, won’t move around much and won’t burst (the pressure release valve is set to 12-13 bars)?

Maybe I could drill a small hole in the centre and secure a metal plate with a bolt and washers?

Would Sugru or PET react with the dissolved cleaning powder?

Any ideas and suggestions are welcome!

Please do not suggest just buying a blind filter, I cannot source them for under 25$/€ where I’m at.