Looks to me like you might only have to take off 1/2 inch. You might not even have to shim - maybe you've got enough take up in the turnbuckles? Though, as the guys have been saying, removing the deck plate and shimming would be super easy to do. I would not use Trex (bits of wood splinters mixed with plastic,) but maybe a piece of Starboard. Actually, I'd buy a piece of G10 from McMaster-Carr. Then again, Rig-Rite sells a replacement foot for the Z-Spars mast (you have a Dwyer, not a Z-Spars) but it doesn't have the same aluminum casting with integral sheaves for internal halyards the way the original Z-Spars did. And what do they send with the new kit? A 2" hunk of plastic that looks very much like ultra-high molecular weight polyethylene (StarBoard). So... My paranoia about UHMWPE is that it can creep under load (so don't use it for backer plates.) This is why I'd use G10 board, which is way overkill. But if it's worth doing, it's worth overdoing.