I have made many of them,I will make a small block that will fit into the cavity where the braib was and screw it itno place from the top.(this gives you a block to screw the plaque into)I screw the horns onto a pieace of plywood,them mix rockhard puddy,( you can buy it in large cans at the hardware store),mix so it is fairly stiff.take a long puddy knife and just pack it in and around the horns and the mounting block and form a skull plate that looks like you want.keep working it till it starts to harden,(only a few min.maybe 30)keep water handy,dip your puddy knife in the water and it will smooth the puddy real well.work it up to and around the burrs.
Give it a day or two to dry. then around the burr,paint about a 1" ring with flat black paint(this will hide any of the puddy you can't cover).then you get black felt cloth and make a cover.lay it on the plate and cut a split from each horn to the back.then cut a circle where the horn will go,when you have it to a perfect fit.
Lay it out put contact cement on the plate and cloth,let dry (about 5 min.) then Very,Very carefully lay it on the plate,roll it down smooth,fold under and put on plaque.