As an engineer, the first things I would try next, an Imperial allen key to see if you get a better fit.
A nice brand new pair of mole grips, you can get a set of 3 sizes from screwfix which are fairly good quality, I bought some a few weeks ago to do a particular job at work ( I have never owned any before, but these were my only option with the job in hand ) item no 4091V. £8.99
Try some shim steel / feeler gauge down the side of your 8mm allen key and bend it 90 degrees over the end of the allen key so that the key pushes it down the hole as you insert it.
Best of luck :thumb: