Looking at ways to reduce tail rocker by 5mm - 10mm on a old 125L board (50cm ofo)
Thinking building it up with epoxy and filler like West System 410 Microlight filler? (heavy)?
Or could I use say 5mm Divinycell, shape it, and glass over the top?
Any suggestions????????

Depends on my much of an experiment it will be......? I'd go with the former . It is nice as if you get the level right and tape around the rails as it self levels there won't be a heap of shaping to do. 5mm is not much - 10 is a lot of fill.....
The latter will still be heavier than you think by the time it is glassed, blended in with some filler and painted (?) buy yeah better than just heap of bog.

What's the project I am curious

Be careful with that desire, not knowing what you're trying to achieve.
Many years ago I bought a board off a friend of mine for next to nothing, he struggled with getting that board to go faster. He figured that it had too much tail rocker, slowing him down, so he built up the tail to decrease the rocker, did a really good job. He taped off the rails and poured resin on the tail, building it up about 3cm, you couldn't even tell w/o looking closely what he did, flared it out beautifully. But it didn't help, got pissed off at the board and sold it to me. I rode it and it felt like it was plowing, the nose was too low while planning, so I took my belt sander and removed the build up that he did, got it all smooth again back to original shape. That helped speed it back up, but when fully planning, and in the foot straps, I also felt it should be going faster, so I moved the foot straps forward, and bingo! The board took off like a rocket, and I was passing the guy that sold it to me, he got pissed all over again.
Turns out that the foot straps were just too far back, that's all it was. It was a semi-custom board built for him, not a production board.

Interesting point Mastbender, the board does plow rather than fly and I do run the footstraps and mastfoot right at the back to try and overcome this, so I best play with foot strap position etc. first

Thanx to Mark for your input also

I'd be spectacle about changing any factory setting ,,,
I have a 117litre board which you think would be a early planer
i changed the fin ,and light sanded the bottom ..it doesn't work unless it over 2o knots still
its just that kind of board ,,,it will skip up to 31 knot very easily..it just needs more wind
than all my other boards ...its a slalom board that needs wind ..the fin made a huge difference .

I agree with the above.l got a RRd fireball 2003 . there's about 3mm of tail rocker aft of fin box.this board flys once it gets 18 knots & beyond.l strapped a Goldwing Tuttle fin in it got 35 knots out of it . sometimes you need the factory tail rocker for the board to release in chop.fins can make a huge difference in the performance of a board

I've successfully done the bog method but as the tail with vee is two panels I bogged 15 mm wide centerline then bogged only 10mm wide around rail edge. When dry I sanded to the thickness blend and vee I wanted. I then bogged up rest of the tail using the dried sanded bog as a guide to drag a straight edge over = Minimal sanding.