Several possible options:
Do you have or have access to a hand-held belt sander? A lot of people never think about them for projects like this. You can tie/clamp the hand-held belt sander to something to hold it steady, turn it on, and then "press" the antler against it. Or figure out a way to "clamp" your antler slab to a table/board, and then hold the hand-held belt sander down against it. For sanding/smoothing some metal parts, I nailed a couple thin iron bars to a scrap 2x4 - angled a little towards each other. I then "wedge" the iron piece I want to sand between those two bars, and use the hand-held belt sander or the angle grinder with sanding wheel. The angle grinder is quicker, but harder to get smooth and "true". You too easily end up with little ... waves ... in your piece.
A lot of wood workers (and iron workers and knife makers) made their own Disk Sander out of a round piece of thick quality plywood. They either attach an arbor on the back, or bore a hole through the center. And then slip it onto the end of a low rpm electric motor and bolt it tight. Then take a round sheet of sandpaper with glue on the back, and stick it onto that disc. Almost instant disc sander. At Jay's blacksmith shop, we used a metal disk made for that purpose, and built up a wood frame in front of it to rest the piece we want to sand on. Cautionary note once again ---- LOW RPM motor! And watch the surface on any wood disk you might use. They can dent/gouge in use, and then give you uneven sanding - especially if you do some more heavy "grinding" on it.
The other option is to use rasps and files to smooth/level/true it up by hand.
And the ultimate CHEAP option: go outside, find a level section of sidewalk with a somewhat "coarse" surface, and start ... hand sanding it on that cement. Um ... do sweep or wash off the "dust" before "higher authorities" see it!
And always be careful of the "dust" as mentioned above! It is organic matter, and can give you a bad sinus infection if you breath in too much of it and don't get it cleaned back out! Far worse than breathing in wood dust!
Just a few humble thoughts to share. Take them as such.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands