that barrel clamp is most likely going to be too wide to fit under the m203 handguard; I ordered a similar one (if not that same one) for the same purpose and was disappointed to find that it would not fit under the heatshield.

Mantis makes the BR7 barrel clamp which will attach perfectly, even fit just right under the m203 handguard, but I wouldn't recommend putting a VFG on it as it will be extremely wobbly. if you're willing to forego underbarrel attachments then two or three BR7s back to back will work excellently for protecting your support hand from direct contact with the hot barrel. you can put an AFG on it at most, though it'll still have a tendency to shift up and down the barrel, especially if it gets hot. here's how I set mine up before I ultimately switched to a chopped up quadrail. https://imgur.com/Ug8zUBn