A very slight scale change and power setting adjustment and voila, it’s made of baltic birch and you can abuse it as such. Thanks for posting.
EDIT: Hmm correct me if I’m wrong, but it looks like maybe you didn’t compensate for the angles in your slot width (non-90 degree angles must have wider slots to accommodate “off-angle” cross pieces). Cardboard can be cajoled into fitting anyway, but baltic (or other woods) will be less inclined to humor you. It’s possible to figure out the appropriate slot width with the law of sines.