Couple of options for "filling" the gab between the blank and cork/real seat.
Easiest and quickest, which works works well is to use masking tabe to build up collars until everything slides together nicely. I leave a small 3-5mm gap between each collar to allow this to fill with glue. The other option is to us a carbon spacer, but unless you have means of turning these, they are a pain.
As for the cork going over the reel seat, you can buy a pre recessed cork ring, otherwise you need a lathe, steady hand and a very sharp and fine chiesel/cutter.