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I'm building up a kitchen windowsill that will be behind the sink.
I need to build up some box-work to mount the tiles on. My first thought was Aquapanel, but its expensive as i'll only use at most 1/3 of a board. I assume external plywood (or even plasterboard) will be fine, especially as the tiles in the bathroom are straight on to regular plasterboard (not done by me)
ply is fine. go for the hard wood one with a textured finish, not birch, so its got a good key to bond to. If its just a small area 6 mm boards should do.
I tiled directly onto plywood boxing in a bathroom and it's been fine for the past 15 years.
it will fine on any ply they do a tile adhesive specially made for wood which helps it move with the wood anyway so you wont have no problems there.
Ply will cost you a lot more than a sheet hardie backer. Just use regular 12.5 plaster board and drywall adhesive to stick it. Double up the boards with a little dry wall addy in between.
If the area will be regularly exposed to water, you should probably give it a coat of water proofer/tanking solution. I use mapei gum but it's expensive stuff and you will only need a cupfull so probably best use hardie backer. Still fix it with dry wall addy though
As for adhesive, use either BAL, Weber or Mapei, however without a trade account you'll pay through the nose for it. I never use ready mix addy, only cement based but the tubbed stuff is fine for a kitchen.