There are Canuck or Turkish pump shotgun parts for sale on various Canadian gun store web sites. They are usually quite cheap. You can buy a forend and tinker. But it is almost impossible to improve the loose fit (due to the simple drop-in slot design) unless you can re-build it in a way that you can add a Remington or Mossberg forend tube (weld it to the action bars maybe?), so you can use Remington or Mossberg forends on it?
I had also thought about adding a plastic sleeve to the inside of the forend for a slightly tighter fit to the mag tube, so it won't wobble so much. But I haven't found something suitable to put on there.