Non shrinking grout...
- http://www.sitebox.ltd.uk
- feb_FBGROUT25
- 25kg bag
- £23.47 inc VAT

Someone might do a "mini-bag" somewhere for about £10, otherwise I
suspect it might sell ok on Ebay.

Some plates come with a large hole in the middle, you do a plasticine
ring, then pour the grout through the hole.

If any studs are projecting, domed acorn nuts might be an idea,
together with some padding. I have a suspicion the pro stuff use a
rubber covering plate moulded to over fit the base. Sort of like a
speed bump rubber. I guess a UV stabilised 1" thick carbon black
rubber sheet square or disc "giant washer" pushed down over the post
would do.