There are boxes like this which appear to be what you're describing:
http://www.mantzel.de/neu/Power-ResonatorQuite an expensive metal box, that one.

The thing you have to appreciate about the multiram system is that it allows the length of the intake tract to be adjusted to engine RPM.
At low RPM you want a long intake tract for best power. At high RPM you want a short intake tract. The multiram system goes some way to achieving this by splitting the intake tracts for each bank of the engine and providing two valves to join these tracts together, one in the plenum and one in the u-shaped duct behind the rad.
This gives the best compromise between low RPM and high RPM grunt, and gives the engine a very flat torque curve from as low as 2500 RPM up to the red line.
Most aftermarket tuners probably don't understand the finer points of what the system is achieving and see it as restrictive, hence these products exist.
Yes, you can probably get marginally better power at the top end by replacing the standard setup with a shorter duct but you lose the ability to tune it to low RPM running.
In a heavy car like an Omega I would regard pull at low RPM to be just as important, if not more, than getting the highest possible "pub figure" at 6K RPM.
But, you have the box. It won't do any harm to give it a try and see for yourself. Can always chuck it back on Ebay if it disappoints.

Kevin