You'll have to 3d a filler piece as well. Jhedsy posted a photo in this thread only a few posts back where he 3d printed a filler piece to use with his picatinny handgaurd.
Here is his photo of the filler piece
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I had a quick look at this https://cults3d.com/en/3d-model/game...octa-handguard and I don't think my adapter will fit it without modifying my adapter for the cyma rail. The block protrusions will get in the way of the smooth handgaurd. The blocks on my adapter sit inside the three picatinny rails holding in in place.
I have seen an AR-15 long aluminum rail cut and fitted to a GSG16 that covered the faux suppressor. It actually looked pretty good. I can't for the life of me find the picture though. The owner must have taken it down. I believe it was one of the long airsoft ar-15 rails off aliexpress. It might be something worth looking into.