I did this tonight
Collimated my 2" diagonal , didn't use shims .
After removing the back plate I fitted a 3mm thick plastic foam material that wants to spring back after you apply pressure .
This was cut out a bit smaller then the plate ' then I screwed it back on but not tight .
Now instead of the laborious task of star testing then adjusting again & again'''''''' I just used a cheshire eyepiece ( the short one without the cross hairs ) you leave the lens cap on & adjust the angle of the cheshire to pick up light that will travel up the tube through the objective lens elements & show a bulls eye pattern through the peep hole .
Without the star diagonal I see a perfect concentric bulls eye , with the diagonal I see two bulls eyes overlapping .
I then start tightening one of the four screws whilst looking through the peep hole , I keep trying different screws until I merge the pattern but I want them perfect so I tape half over the 45deg silvered section of the cheshire , now I can see that the two halves of the bulls eye need to be finely touched up to make it perfect .
Also fixed up my inch & a quarter diagonal that was out of wack , the mirror in this was fixed to the back plate so had to fit two rails of the 3mm stuff up the sides to be able to have some shift , anyway got that spot on also .
Have Fun
Bobby