Hi Warrick
I have a wood cap on my pier but it is in an obs and not exposed to the elements. I have not had to adjust the polar alignemnt for over a year so its pretty stable (if you use the right kind of wood). I think a propper steel cap would probably be the best solution but they are pretty expensive.
The wood one I have is pretty substancial. I went to a timber yard and they had precut hardwood timber about 1.5m in lenght and 4cm thick. Used for making stairs. I got a piece that had a damaged corner for a princely sum of $17. Cut it down to size and mounted it to the top of a concrete pier.
It was easy to work with. I simply cut to size (allowing some extra length for placing eyepieces when in use). Counter sunk some holes half way through and used big nuts to secure it to the threaded rods comming out of the pier. That left a flat top to which I bolted the meade wedge.
You could prolly use something similar but would need to weather proof it as it will be exposed to the elements.
Nice thing about that is that its cheap and easy to work with (I also cut extra round bits out of the offcuts to add extra shelving on the sides and to add a bit of a finished look). If it all went bad, it was under 20 bux but its has not moved in nearly 2 years.
I have attached a couple of sample images. The snap was taken with my phone, sorry for the cruidity of the photo.
In regards to pier height, in my opinion it really depends on so many factors. Type of scope, your height :-) But Basically the way I see it, if you want to look through the eyepiece for an extended period and the scope is pointing up, try to have it so at that posi it is comfortable to get to the eyepiece. you dont want to have to get on your knees to use the scope or always have to use a ladder. Not so critical if you are imaging as you will have a camera in there most of the time.
Regards
Bill D
