I inherited a greenhouse on a plot last year and it's been a steep learning curve! I bought
this pair of saw horses from B& Q, plus some 2 x 1 wood.
I basically created a bench shelf along one long side of the greenhouse by resting two long pieces of the wood across the saw horses, and adding cross-pieces of wood so that they braced the sawhorses. Let's see if I can do a little diagram:
The ------ is the long pieces of wood, which are parallel to the long side of the greenhouse
The ^ symbol indicates the top of the open sawhorse
The | symbol indicates the two pieces of wood which are either side of the open top of the sawhorse, stopping the top from sliding around
|^| |^|
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|^| |^|
|^| |^|
|^| |^|
|^| |^|
-----------------------------------------------------------------------
|^| |^|
I then tied on some spare wire shelves on top of the two long pieces of wood (I'd deliberately checked the depth of those shelves and ensured the long pieces were that far apart, and Bob's your uncle
If you felt like it, you could cut and screw pieces of wood as crossbraces instead, on which you put the plants etc. But that's a lot more work and I had the wire shelves kicking around. It means it's easy to disassemble if you want to put it away later in the season