If you want to make up your own manifold for motorbike quad throttle bodies you might be able to get a flange from Clyons, if he's got any left. He had some made up a while back. (Hopefully I'll use mine soon, just about got all the bits ready.)

I'm going mandrel bent mild steel 1 1/2" with CBR1000RR throttle bodies (around 44mm). Mine came with an airbox, original rubber flanges, trumpets, inner and outer injectors, throttle cable, plumbing etc for around $100-, it was a good buy (I think). The flanges taper down just under the 1 1/2" pipe so once I've made up the piping and flange plate for the throttle bodies I can port match it. Don't expect fast progress though (and hence pics), takes me for ever to get anything done. The 1 1/2" exhaust pipe is very close to the CSA of the a15 port.

There was a 1000 up for sale a while ago with a similar home built set up. Depends how much work you want to do yourself. If you have to pay for welding/ fabrication it would be cheaper to buy a commercial set up like efi hardware (for that you might save a quid if you can get a dual webber manifold secondhand).

Do a search on this site on quad throttle body, itb, motorbike carbs, webber side draft etc. There's a lot on this topic floating around.