Pool Heater addition inline

[caddywhompus]

I have a Coleman 16' x 48" pool that I am adding a pool heater to. I am having issues finding the right fitting to create a nice watertight seal.
The threads on the pool pump outlet are 2" NPT male with a gasket.
The threads on the matching flex tube are a 2" NPT female, with a flat surface for compressing the gasket.
It is like a union pipe fitting, but I can't find a union pipe fitting of the right size to mate with it.
Original setup:
1) Pool pump -> Flex hose
2) Flex hose -> Pool wall

My proposed setup:
1) Pool pump -> Rigid hose I made with barb fitting adapaters and hose clamps
2) Rigid hose -> Pool heater intput
3) Pool heater output -> Flex hose
4) Flex hose -> Pool wall

The issue is really at two points:
1) Between pool pump and the hose I made. The host end is adapted to a 2" NPT female thread. This has no flat surface inside the fitting for the gasket to compress against because of the company's proprietary adapter.
2) Between the pool heater output and the flex hose. The flex hose is connected to the 2" NPT male thread of the pool heater. This has no gasket to seal between the NPT threaded adapter and the company's proprietary adapter.

Any help you could give would be greatly appreciated.
I have spent a long time scratching my head as well as driving back and forth to the hardware store.

I have pictures attached.

Thanks

 

[WorthFlorida]

Usually pool heaters are for in ground pools with a permanent installation using PVC pipe all around. But from the pictures I cannot really tell with one is giving you the problem. All fitting seem to be unions. Also, using pipe dope is a not recommended on plastic pipe, use Teflon tape. Pipe dope is very difficult to clean off and contact with water can cause irritation to some people, especially if it contacts drinking water. Pipe dope was invented for iron pipe threads, usually for gas lines.