Anything should work fine, except the ones with hardplastic - as they have mostly cutoults and the additonal 3-4 millimeter your modded 3DS is thiker might be a problem. for exaple this would not be that good here: http://www.amazon.de/Protection-Case--c ... s=3ds+case
I have already got a hard case for my N3DS XL but that is being used, all I have left is a pouch so I want something more durable. Maybe one of my old DS lite cases will fit, they are 3rd party and rather big in comparison.
I'll make sure to leave feedback once it has arrived.