Override a few keycodes with XKB

I want to shuffle a few keys around with XKB. (Why? On a laptop where some keys are inconveniently located.) I currently use xmodmap:

keycode 110 = Prior
keycode 115 = Delete
keycode 112 = Next
keycode 117 = Insert
keycode 119 = End
keycode 118 = Home

Instead I want to use XKB and assign different symbolic names for certain physical keys, rather than assign different keysyms to certain keycodes. (This is why.) I want keycode 110 to send PGUP instead of HOME, keycode 115 to send DELE instead of END, etc. The rest of the configuration must not be affected (so PGUP is to keep sending the keysym Prior, etc., and all other keys remain as they are).

How can I change the assignment of these specific keycodes? I'll load a file with xkbcomp somefile.xkb $DISPLAY, what do I need to put in somefile.xkb?