I have a miniature gelding I raised that cribs(wind sucks). He is an "A" type personality, and I often wonder if he might not have started this had I provided him with LOTS of toys very early on. He has NEVER been confined to a stall, except at the OCCASIONAL show, and overnight in winter in really BAD weather; he has a 12' X 36' run attached to a 12'X 12' stall as his living arrangement-and is occasionally turned out-if there is grass to graze(unfortunately, not often here, esp. with the last few years of severe drought...)-and sometimes, in my large arena-and as he is one of my finest, he is also driven. He is not the 'worst' cribber I've ever seen, but is pretty confirmed in doing it. They make a Weaver cribbing collar in foal size, which,after punching extra holes in the straps, does fit him, and which I have on occasion used on him, for relatively brief periods of time--and it DOES work to prevent (he is a medium build 33 1/2" horse) him from the action, but he 'looks' MISERABLY uncomfortable wearing it(it almost certainly is NOT as uncomfortable as it looks; i.e., when a horse puts its head down to eat/drink, the collar is 'automatically' looser, and in no way interferes with either eating or drinking)-but over time, it does tend to rub(even though I made a fleece liner for the 'over the poll' strap). Bottom line-I decided not to obcess about it myself. He does sometimes appear a little 'inflated', but has NEVER had any health issues. When I was showing, I might have him wear the Weaver collar for the week or so before, and during, the show -- then removed it afterwards.
That said, I HAVE taken measures to MINIMIZE how successfully he is able to crib--mostly to help keep him from excessive wear of his upper incisors. My runs are constructed of panels welded of 1" square tubing. They are 'lined' with welded 2"X 4" wire, mostly to keep the horses from rubbing their manes out reaching through trying to eat any blade of grass, but also to discourage any dog/coyote from easily coming into the runs. I have ONE run that is 'lined' on one side and the end with mink wire--a VERY HD mesh with 1" openings similar to chicken wire-that I'd gotten from my dad, who once(MANY years ago, so no flames, please--I don't believe in wearing fur, either!) raised mink-and had this one 50' roll of this wire left. So, I placed this horse in this run. This left this run with one 36' side 'lined' with the 2x4 wire--so he was basically prevented from having any place on the sides of the run to crib--until he figured out how to crib sideways on the wire itself! So, I added plain chicken wire to that side--but it is really too lightweight, and has become broken/bent, and needs replacing. Where a bar of the panels became exposed, I then cut through one side of some semi-rigid, black "PVC" and slipped it over that panel. That is the ONLY place this horse can crib in this run, and he can't do it very efficiently. I also gave him a low profile, HD plastic 'water trough' as a feeder; he can't crib on it, as it just tips if he tries-plus it's too low for 'successful' cribbing--same w/ his water bucket. It is my intention to see if I can buy a roll of 1" x 2" welded wire, and redo the 'liner' of that side of his run. I don't THINK he can get a grip on that, even sideways,to successfully crib....
With wooden rails, the idea about cutting fairly large diameter PVC in half and placing in 'on top' of any rail the horse is cribbing on, seems like a good one to me--seems like you could just drill holes and screw down through the 'half-round' of PVC into the wooden rail at intervals, to keep it in place--and yes, it would need to be large enough in diameter so that the horse really couldn't 'get his mouth around it' successfully. These kinds of measures, combined with plenty of 'moving around' room, MINIMAL close confinement, offering toys and ANYTHING to keep his mind occupied, and plenty of turn out/exercise, should hopefull help, perhaps help a LOT. Fortunately, my horse(now 13 years old)is not the MOST obcessive cribber I've ever seen; the TRULY obcessive ones WILL find a way to continue it, to the detriment of their health and well-being--but with ones such as mine(who will frequently go and stand quietly asleep in a corner ), I can testify that it IS possible to successfully deter, or at least minimize, this activity, and without devices that are a hassle to either horse or caretaker--or both!
Good luck!
Margo