Rocket mass heater outside

Hello everyone been doing a lot of reading on the sight and on the web trying to find a good design for an outside RMH.  I live in northern Michigan so we get some extreme weather changes.  The RMH would only be used from spring to fall. I would like to have a bench with it so people can sit and stay warm on cool days or night while enjoying the outdoors.  The other aspect I would like to tie into this would be a pool heater for are swimming pool.  

My intention thought was to build a J style
rocket stove. And using some type of tubing in the terminal mass to take heat for the pool. I was thinking pex tubing would be good in the mass.  The reasoning behind using the mass to heat the water is I would lower the chance of any kind flash steaming and I would not affect the efficiency of the burn chamber.  
I know I would not get as much heat this way but any extra heat in the pool is good.
I am looking for any information on what to build it out of that would hold up to the weather we get here. Or if anyone has a good design for this. I want to use a rocket stove. So there is no extra cost. I can get wood that is not a problem.
I have never built a rocket stove so I am not sure what all to do. I am willing to spend some money on this but like most people money is tight.  
If anyone has any questions about it I will try to explain better. When I get to build it I will definitely share my results. It is currently winter here so won’t be till spring/ summer befor I get to build it

Thanks for your time and help

Welcome, Wade! Your plan sounds good. Unless you have a roof over the bench (kinda interferes with stargazing), you won't be able to use cob. I think a pebble style mass would work for you, light on the mass, or maybe a bell bench with stone slabs or pavers as the top. Pebbles would be more amenable to running a water tube in moderated temperature.

Hi Wade;   Welcome to Permies!
I second building a bell bench with no roof.
Brick sides with paver / stone top.
Now your J tube core and barrel I suggest a roof over it  for snow protection and so you can use it during wet weather.

I suggest building an 8" J tube .
Firebrick construction rather than ceramic board.
Morgan superwool  for a 5 minute riser.
A 55 gal barrel with a removable lid.
Firebricks for the core.
Clay bricks to surround.
Fireclay
sacreet brand #100 sacks of  commercial builders sand.
And more...