I have the FN one and it would be pretty difficult to make a copy of it youself.
But for DIY, all you really need is a allen wrench and a penny or a dime.
Use the allen wrench to loosen the hex screw then use either a penny or dime to turn the outer screw. Once its in the location you need, then use the allen to tighten the hex scew up.
edit: The locking mechinism is very simular to many infant/childrens toys.
Materials required: Highlighter marker cap (Standard sized highlighter) of ~0.54" OD
Tools required: grinder
Find a cap that is about 0.54" OD. Grind off enough material so that 2 notches about 0.1" wide remain. Keep trimming until it fits the adjustment bushings on the reflex sight.
If you break one cap, find another one. A whole pack of highlighters or markers can be bought for under $2 USD. It sure beats $28 or $50, and we only really zero out the sights a few times at most since the PS90 was never intended to be a target/precision rifle.
Why plastic? It's easy to work with, it won't scratch the weapon, and the highlighters/markers are usually non-toxic so it's safe to work with. The marker caps are soft enough to grind through quickly so watch the RPMs. The marker caps are also hard enough to turn the adjustment bushings.
I love working with metal until I get steel shavings/chips embedded in my skin, or covering the garage floor.