Here is an ugly piece of code that shows how to remove the sideborder. Of course there is much more to desire if you want to use it in a demo, you probably want sprites and perhaps graphics in the open area as well. This just shows how it worx.
Please note that the code is assumed to be started from a normal basic screen. Particularly the $d016-register is assumed to be $c8 at start. Have fun! / HCL
sei loop1 lda #$1b ; Set y-scroll to normal position (because we do FLD later on..) sta $d011 lda #$3d ; Wait for position high up on the screen cmp $d012 bne *-3 lda $d012 ; Ugly way to get stable raster and #7 ora #$18 sta $d011 nop ; Wait some cycles to make the loop work fine nop nop nop nop lda 0 ldx #0 ; Set 0 to counter loop2 clc ; Do FLD to avoid badlines lda $d012 adc #4 and #7 ora #$18 sta $d011 ldy #5 ; Wait a little dey bne *-1 nop dec $d016 ; Set 38-chars width of screen -> remove sideborder inc $d016 ; Set back to 40-chars inx ; Increase counter cpx #$80 bcc loop2 ; Branch if counter less than $80 jmp loop1 ; Next frame