Well I was a bit slow to reply to this post, so already my learned colleagues have answered all of your questions James, re: the spin on filter debate, most of the big motorcycle manufacturers use spin on type filters on modern bikes now, I just bought a couple of fancy chrome filters for Suzuki's that I might adapt my centre bolts to fit on future coolers, these filters are smallish in size, but the cores would be a tad larger than our OEM Honda items. If there's a concern with aftermarket filters, then I'd use a genuine Honda OEM spin on filter?
I'm not convinced though that there's anything wrong with the popular Framm filters, we don't see many Framm's here in Oz, Ryco is the more common brand here, and I use them on my cooler adapter with no problems, and as far as longevity goes, I've been using Ryco's on my wife's old Ford for the last 13 years and it's done well over 400,000 Kilometers (250,000 miles) without the head ever coming off. Of course if you're that worried about spin on filters that you continue to risk running an ancient aircooled engine in summer without a cooler, I also make an adapter so that you can use your OEM filter and cover.
Regarding price, well you can always find a bargain on ebay for a used cast "pot metal" cooler adapter, with no guarantee as to fit or quality of construction, or even whether it works, and if you want one let me know, I've got several hanging in my garage that I'll sell you cheap. If on the other hand you'd prefer a fully machined 6061 T6 alloy hand built adapter that does work and will probably outlast your bike, pony up the dough guys, and I'll make you something that you'll be very happy with. My adapters cost $75.00 plus shipping, and I'm always surprised when some of you guys baulk at that price, I reckon for every 10 enquiries I get, only 1 person actually follows through with a purchase, so what price peace of mind? Obviously a lot less than 75 bucks, ha ha! Cheers, Terry.