No problem.
As far as I know, there isn't a greddy listing for rb20 plenum I could be wrong through. R.I.P.S in Nz make a tidy little number for rb20's, and I have had a look at the "china" rb20 plenum's you see on ebay (look the same as the rb25 ones they have listed, just suited to rb20). They appear to be made well MINUS the fact you have to machine the facing to get it to sit right and you'll need misc hose's ect.
A trader on secret drift has them, or had them. Brings them over in bulk thus cheaper.