Canvas? Interesting. Haven't heard of that being used in composites before? 

It's a heavy cloth that saturates with resin...any reason why it wouldn't work?

Well what's the modulus of canvas? Denim is a cotton fabric and will absorb a lot of resin but it doesn't have any strength. Don't forget that resin itself is very weak, it's the fibreglass that makes things strong/stiff. 

Depending on the type of "canvas" it may work just fine ..   It will add some strength, but not nearly as much as glass-fibre..

What you should have bought at the local hardware store is  "Wall -glass mat"  of the cheapest type without any primer or binder in ..

Works a treat  with Epoxy ... and its cheap ..  No idea how it would work with Polystyrene though- --

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

I think it might be useful to put reinforcements also horizontal and diagonal to prevent flex as you wrote

i cant wait to see more. This is a project ive often thought of doing

That looks excellent, congrats and well done.

Yes, it's a little flexy from corner to corner, but not so much that it will be an issue.