I made an envelope bag to fit in my Gansai watercolours and a colouring book to carry around. I measured so carefully where to put the second piece of velcro type fastening - and I put the hookie side down instead of up - see - we all do it ;-) I very carefully unpicked it and turned it round.
Don't ask me for sizes - I just used to box as a template for the big front - then cut envelop shaped flaps for the back. It is lined with some fabric that was gifted to me about 3 years ago - the bias is from the pattered bundles we have in the shop - and the red pompom is from the the felt trimmings that I am selling off
It takes my box and bits very comfortably.
And I have actually used them this week