right tool: DIN wrench.
available at glacier tanks.
https://www.glaciertanks.com/din-fittings-round-nuts.html
maybe @precisionnick can share the tool they suggested for tightening DIN sightglasses back in 2015. I won’t.
Picture below are the oil filter wrench I picked up at St Vinnies, and a decent approximation of a DIN spanner. I used the oil wrench to get the sight-glasses on that PX1 tightened down for my initial pressure test when the PX1 first arrived.
I think it took me another week or two to run into that ($6?) spanner. Which is (pretty close to) the right tool for the job.
looks to be one of these Forestry Spanner Wrench - Double Ended (so not an actual DIN spanner).
if I recall correctly I picked mine up at Pacific Rubber.