The big spring works well.
A few years back a friend of mine had a SolarForce and I sent him a M61. Him not being a flashlight guy tried a red o'ring and it worked. I just think the gasket Gene sells (and calls scout head adapter) just looks nicer than an o'ring.
I think some have used alluminum foil wrapped around the M61 with success too and say it actually helps conduct heat better. But mine has a 25 lumen module so heat is not an issue.
Some say the firm push required on the button combined with being sunk below the shroud is an issue. I just use a finger tip to push the button like I need to on my SureFire backup. To be a sub $20 host it's actually a real nice value. Tough coating, glass lens, excellent machining, stainless bezel ring, grip rings, anti-roll shaping, double o'rings for added protection, oversized barrel for 18mm cells, holes drilled side by side so a lanyard doesn't affect tail stand, provision for paracord, comes with a twisty tailcap separate. A lot of product for not a lot of money. I prefer a black tailcap button (over the orange) but that's an easy fix. You can use a P60 or D26 like a Sportac in these. It's a wonder these aren't more popular.