How important is an exact fitting helmet to the helmet users?
To me, the answer really depends on the environment and type of diving conducted. I can understand why cavers need a properly rated/specified impact-resistant helmet; especially if their activities include 'dry' passage, abseiling an climbing... and/or the risk of loose rocks falling on them before they're submerged.
For wreck diving, there are none of those 'dry' issues, and underwater the velocity of impacts is substantially reduced (even if momentum/inertia isn't). This doesn't necessitate such a sturdy helmet - so I find it acceptable to use a 'lesser' helmet and save money.
I made a bunch of helmets a few years ago; to equip myself and my wreck students. My own helmet was adjusted to fit and 'locked down' with an ample wrapping of duct tape around the inner liner. My student/loan helmets remain adjustable.
DIY Sidemount Wreck Helmet Project | Andy Davis Technical Diving
What was important was to remove the foam liner on the helmets as these caused excessive positive buoyancy. It was an easy job to replace those with plastic, adjustable liners. I had bungee on my helmet to replace the nylon/QR chin strap. I initially left the nylon/QR chin strap on the student's helmets, thinking it'd be easier for them to remove/replace, but I saw students have more difficulties with them, so after a while I changed them out to bungee chin straps also - they
are far easier.
Using a bungee chin strap, all that's left is a quick adjustment of the inner band and the helmets fit snugly. I get feedback from students once they used the helmets; and in every instance they prove no distraction on the dive. A couple of dives and they're forgotten in the water.
The design of helmets I used seem to work very nicely for sidemount/wreck. Most of that initial batch of helmets are covered in scratches now - indicating successful protection of divers scalps from rusted metal. I've had rusty metal splinters embedded in my scalp before (
when not using helmets) and it's not nice. The design offers good protection from scratches and collisions, but also does little to interfere with freedom of movement of the head (
especially the ability to crane the neck upwards when operating in proper trim).
As I normally introduce students to helmets on wreck courses, we have ample chance to practice long-hose air-sharing in open water dive/s before penetrating the overhead. This includes black mask drills. After sufficient repetitions, the students adapt donation technique slightly so that helmets cease to potentially interfere with donations. Really, all it takes is a slightly exaggerated motion.
The only draw-back I've found with helmets is when undertaking very tight penetrations. This probably isn't an issue for most wreck divers - but I've found the helmets are sometimes the limiting factor in spaces I could otherwise crawl through. Especially when negotiating restrictions feet-first, where the sidemount tanks are partially-detached and pushed/pulled. Turning the head to the side lowers the profile somewhat - but there are times when I've removed the helmet to get through particularly tight sections... or removed the long-hose cylinder for a 'drag through' (hose still in place, cylinder lowered and clipped to an ankle bungee).