By the time you want to make a cut that far into the board you can use a straight piece of timber or better still a rail clamped to the board. Something like a spirit level might work.
Before track saws that's how a lot of carpenters cut sheet goods on site.
OK. I'll keep an eye out / do some searching during my afternoon procrastinating.
It's also annoying that someone doesn't just make a longer version of the stock Makita one. It's not exactly complicated engineering.
EDIT: found this one for £50 - that's the one right?
https://www.ebay.co.uk/itm/Kreg-KMA2685-INT-Rip-Cut-Metric/233508155864?epid=20021623683&hash=item365e2bb1d8:g:prwAAOSwXy5eV-Nk