Non-warping board material?

[andyroo]

Need to re-make the vertically hanging lid-door to the 250 in-wall.

Current is 3/4" marine ply at monumental cost & weight, and it (f@$%ing) warped - nobody listens to the PhD with 40+years experience, for reasons I'll never understand. Maybe my consulting rate is excessive...
It was made-to-work with a series of flat tiber braces that are now causing head-bash injury, and will almost certainly catch, retain & crisp-to-kill smaller jumpers in the pending stock re-shuffle. Then there's the 80Lbs of timber on my noggin while I try to work in there...

Thoughts & Options?
Need something that won't warp, lighter & about the same painted texture as rendered concrete (or ply).
I'm thinking 1" styrofoam with a layer of (epoxy) fibreglass on the wet-side (remembering my surfing days). It likely won't be strong enough for another arowana, but I don't think I'll keep anything so big in here again.

 

[Ulu]

30 years ago I was an engineer at a company that put huge Styrofoam panels on the sides of buildings, and they were covered with various plasters that make them look like concrete or stone.

To make the stuff stay there forever on a big exposed panel, you have to cover the Styrofoam panels with a screwed-on poly mesh first, and it is attached through the Styrofoam to steel or board the back side.

We only put the plaster on the various exterior surfaces, and the Styrofoam was always attached to a backer.

But as a small door you can put mesh and plaster on all 6 sides if you wish.

We used to apply an expensive acrylic plaster called Dryvit, that came in 5 gallon buckets already mixed, and it had a scratch coat/base coat/color coat system. There were less expensive plasters that only required one or two coat application, but much thinner and less textured. Also fewer choices of color.

But if you watch some of the Dryvit online application videos you’ll get an idea about how this process works with Styrofoam and how you might want to do something similar (but less industrial) on your door.

 

[andyroo]

Googling Dryvit, thanks Ulu
I'm guessing that the styrofoam didn't warp, or the cement would have cracked(?).

A local sign-maker supplies importer has 2mm & 3mm PVC board in 4x8 sheets. That should be waterproof, but might I expect warping with the temperature difference (22~24C on the outside & 25~27C on the inside with the lights)? PVC works well with epoxies, giving opportunities for light bracing if/when "things go sideways" and will take a spray-glue for a paper outer render/primer for a drywall'y finish, whereas polystyrene is much more chemically sensitive/reactive. Thoughts?

 

[Ulu]

You won’t buy real Dryvit, because you can’t normally get small quantities, and when you do they are probably stale leftovers. But the concept of cloth reinforced, acrylic fortified plaster, on styrofoam is a good structural choice.

Much depends on the look you want.

You can buy the plaster and the acrylic fortifiers at Lowe’s and mix it up yourself. You can add texture and paint it any color.

With Dryvit the color is mixed right in the plaster. If it chips it’s still matching the color.

 

[fishdance]

Look into gas struts to open/raise heavy lids easily & safely.

 

[Ulu]

The only issue with struts is you must reinforce the panel a lot where they land.