Hi d man,

I know it's plastic your after not sheet metal, but it may benefit you to draw it via sheet metal features as it will give you the option to obtain the flat pattern if needed.
If you sketch a oversized square the click base flange/tab and apply thickness and guage table (solidworks standard one) then create a sketch (single line) horizontally across the square and click sketch bend you can the fold the piece to suit the angle on your pipe. If you then insert this into your assembly and mate it fully you can then look normal to the pipe. Edit the new part in place and create a new sketch on the correct plane. Create an offset from the outer diameter of the pipe to give you the weld surface and then complete an extrude cut (with normal to selected) through all cutting the outer portion of the new part away. This should leave you with your desired part. Hope this makes sense.