Size of ally tube for bunk...

[LeeME3]

Right I have a bluebird ally and canvas cab bed for my front cab. It works well and I have a spare canvas big enough to make an adult bunk - for a 10ft caravan I tow behind the T25 sometimes. Question is what size ally tube should I go for? I reckon an outside dia of at least 30mm with a wall thickness of at least 4mm. Length will be 195cm max supported weight 12st (it's really for younger teenagers but I'd like a margin). Continuous length ie no need for a hinge or join. Also, tube or square section?

Thoughts?

Thanks in advance!