None that I know of.
The LAR-8 design predates the existence of Magpul, let alone their magazines. Everyone did their own thing when it came to developing the larger platform AR-style platforms. Commonality of any components from different manufacturers (except those using differently named versions of the same model) is largely by accident rather than intent. The LAR-8 is designed around the FAL magazine (inch and metric) of which there are millions around, but due to circumstances of age, manufacturing, country of origin, etc...not all are fully compatible with every LAR-8. That was the genesis of our own poly magazine for it...still FAL-based, but largely dedicate d to the LAR-8.
Even a new lower wouldn't help as I don't believe theupper would work with a DPMS-pattern P-Mag either.
Sorry.
Steve/RRA