The main problem I had is that lekku fit onto everybodies heads differently, so I had to do helmet templating of the rear of my head in the mirror because that was the only way to ensure it was fitting me. You should have less issue as your head piece would more than likely be a full head so it could be put on a stand and worked on.
Awww, shucks! I'm glad it worked out, but it must have been a pain in the tucus!! I agree, I have 22 inch head male mannies. I need to buy, at least, two more. I'm running out of room to put project creature costumes on that aren't finished yet. I'll see if I can't make a base off of the foam-work of my creature head and then use it to basically round-up the measurement so it can fit around a furred mascot head and not just a foam base.
Here is what I have, so far, as far as the helmet design goes.

Vents! Vent the crap out of EVERYTHING, for the love of being alive and not dead from a heatstroke!
Is this all right as far as the proportions, go? The vents will be an easy to breath through "Mesh", but not the kind you can easily see through. I really want to wear my helmet, even over the creature head. I have an idea to make it easy to come on and off and it has to do with magnets. The helmet will basically, still be a "Two Piece". A faceplate and then the back that houses the ear guards.
For the sake of my physical safety and my physical condition, so far this year, my mando will probably have a lot of hidden heat guider-outers to make her more bearable and more comfortable. been learning my lesson to try and install more vents in the creature costumes, themselves and even the armor. I have some ideas for some "Cool-looking" vents that could go in the leg gaurds where her "Thrusters" are. Those are probably going to be heat, expelled vents, in reality, but since nobody can intentionally see up at that angle, they look like thrusters.
Let me know if anything needs to be fixed.