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The beholder is Intellectual Property of WotC, so Paizo won't be able to publish any official rules for them.
You can flip open an old copy of the 3.5 Monster Manual, though, and apply the Construct type to the write-up on p. 26-27.
It'll gain 11 hp due to d8's becoming d10's, lose 44 hp as it's +4 Con mod goes away, and then gain 30 hp as a size L construct. BAB won't change. It's Will save will drop to the poor progression, and it loses it's Con bonus, so saves are Fort +5, Ref +5 and Will +7 (after Great Fortitude and Iron Will are factored in). Skills won't change. It gains a boatload of immunities, as part of the Construct type (summarized on p. 307 of the 3.5 Monster Manual).
It gains two more feats (I'd use Toughness for one of them).
Add CMB +9 and a CMD of 21, and you're pretty much good to go.