I found that neca ne neceris means "kill lest you be killed" and would like to modify this to "hunt lest you be hunted." It looks like venor is the verb for "hunting" but I'm not sure what the proper form would be for this phrase.
The most obvious verb to use here is venari, which you've found. The difficulty is, that it's what's called a deponent verb – it has, for the most part, only passive forms; and these are, for the most part, only active in meaning. So it isn't possible to use both active and passive forms of the verb, as is done in the English sentence that you're translating and in the Latin sentence neca ne neceris.
Still, there are many alternatives.
One is to use two related nouns: venator, which means 'hunter' and venatus, which can mean 'animals caught in hunting' (which I don't think is exactly the same as 'the hunted,' but you may decide that it's close enough):
aut venator aut venatus
(Be) either the hunter or the hunted.
venatus nisi venator
(You will be) the hunted unless (you're) the hunter.
You could also do something similar with the nouns praedator, which can mean 'a person or animal that goes after prey, hunter,' and praeda, 'the prey (of a hunter, wild animal, etc.)':
aut praedator aut praeda
(Be) either the predator or the prey.
The advantage here is, that the English noun predator is recognizable in the Latin noun praedator. Plus, praeda strikes me as closer in meaning than venatus to 'the hunted.'
(There is a verb praedari, but it, like venari, is deponent and so presents the same difficulties as that verb.)
Another, more figurative approach would be something like:
tu retia tendas, ne tibi tendantur. / retia tende, ne tibi tendantur.
Spread hunting nets, lest they be spread against you.
(Tendere is pretty much the standard verb that's used in this context.)
retia tende, ne [ipse] inter retia venias.
Spread your hunting nets, lest you [yourself] come among hunting nets.
The ipse here is optional. The inter retia venias bit is based on Vergil, Aeneid 10.710, which is part of a simile where the character Mezentius is compared to a hunted boar.
Or, in letter 1.6, Pliny the Younger writes about sitting at his hunting nets (ad retia sedebam); so a very pithy version could be:
aut ad retia aut inter retia
(Be) either at the hunting nets or among them.
There may also be some sort of ready-made sententious phrase in Seneca's tragedy Phaedra (a.k.a. Hippolytus). The play opens with Hippolytus singing a hunting song, and there are verbal echoes of that song when Hippolytus's own death is described at the end. Perhaps some character, or the chorus, remarks about how the hunter became the hunted. I don't remember off the top of my head, but I'll try to poke around later.