Split focus screen for K100D adventure

The last attempt to modify a focussing screen to put in the K100D went rather well.
Here I am ready to do another one.
Thought I would try for a diagonal split as the screen is big enough to rotate before the fatal trimming.
The donor this time is a super program that has seen better days.
First issue is the blinking screen is glued to the frame
Any clues on getting it out much appreciated
Pete

I'll probably try scoring the glue down with a scalpel first. Want to avoid twisting or levering as the 30 yo acrylic splits easily.
Pete

Yep the scoring went well and soon the screen eased out of the frame.

Ok the screen is taped up and ready to check the 45 degree plan.
mmm good in theory but not enough oversize to get a 1/4 rotate...
Is it worth it ?

I decide not, so the marking out and sanding down will be next
bit of a shame but the split is going to be so welcome,
I don't mind it being horizontal
As you can see the tab on my first attempt was pretty rough.
Will attempt a tidier job. The old belt sander is an order of magnitude too big really

Progress is slow as I contemplate which woodworking tool I can go at this with
Noticed it seems thicker than the standard screen.
Might have to leave out the shim which might lead to 'issues'.
Will have to wait and see.
Sort of sad to see how much view area we lose with DSLR
Pete