I've been extolling the virtues of slab reefing for some time now, but am in a minority amongst Trapper owners. They all like their roller booms!
There are pros & cons for each system. I like the set of the sail & believe that it is easier to put in a reef when it's blowing, with slab reefing. (The majority will disagree & point out that it is easier to use the roller reefing when short handed).
You need to talk to a sail maker to put in the cringles. I only have 2 sets, because that's all you can realistically rig lines to. The sail maker will probably advise on the positions. In addition I have a Cunningham cringle about 6" up from the Tack which i use for luff tension with full sail. You will also need a couple of cheek blocks & eyes fixed in the correct positions on the boom & some means of cleating off the reefing lines so there is a bit of work to do. If you are keen to do this let me know & I can E-mail you with some photos of my set-up.
Finally a warning to all Trapper 500/501 owners. I enquired recently about replacing my old roller reefing boom with the same design from Southern Spars in Southampton. Apparently RR booms are pretty well unheard of these days. A new (non-RR) boom will cost ~ £350. I'm not sure if that includes the necessary hardware for slab reefing.