Here is a picture of my OR with the Troy MRF-C. Note the gas block position and realize the gas block is the proper place for a folding front BUS. Mounting it behind the gas block, on a quad rail will shorten your site radius.

The MRF-c is a free float quadrail and the BUIS are Troys. There is actually daylight between the scope and rear folding Troy that is not apparent in this picture angle.

Point is, you are better off buying a front BUS in metal than trying to adapt a Magpul, polymer site to a carbine length quad rail because of the shortening of the sight radius.

Hope that makes it a bit clearer for you and shows your options.

Also note this quad rail has eliminated the D ring and looks like an OEM install without the D ring showing as on drop in types.