AR upper with a relocated operating spring

[kanton]

Does anyone make a kit or have plans for an upper with a relocated operating spring and rod, on top of the receiver surrounding the gas tube?

I know Allan Zitta at ZM Weapons makes an expensive upper and folding stock kit but I would like a cheaper alternative that I could retrofit to my existing uppers for building a bullpup or pistol without the operating spring sticking out the back of the receiver.

Thanks