How 'alien' are you going ? There's a bunch of commercial franchises to avoid, an FX budget to consider, the B-Movie trope of 'actor in rubber mask and padded suit' to embrace or shun etc etc...

Depending on the view-point, you may manage with 'muppetry' rather than improvised animatronics...

Is there a local Cosplay Crew you could co-opt ??