Remedial trap for a shower

I have a shower tray with an outlet which runs pretty well straight out to a 40mm outlet then to a waste pipe. I thought the outlet had a trap in it, but it doesn't, and we are getting occasional drain smells, (and occasionally worse...) through the waste.
The piping is still accessible, and before it gets closed off I'm wondering whether there is a trap I can put in line into the pipe to sort this. Once the wall is closed it wouldn't be possible to access it.
Any thoughts?

 

Show us a pic of what is connected under the tray.

 

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