I feel like if I screw it all together enough and use a brick like pattern that two sleepers on top of each other will be pretty strong.
Have a look at Toolstation item 42827, (2 x 32 x 32 x 300mm galvanised angle), in each internal corner, drill the holes out to 5mm to accept TS 14627, (5.0 x 80mm Torx woodscrew), with 3mm guide holes about 20mm into the sleepers.
Sounds like a good plan. Would you put them straight on the soil or some gravel for drainage?
@mespilus started
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I feel like if I screw it all together enough and use a brick like pattern that two sleepers on top of each other will be pretty strong.