Well I went ahead and did this mod, fabbing my own set-up from some commonly available aftermarket parts. It's running great - no excess noise, and the engine seems to breath a little better than the stock airbox set-up, esp. with my freer-flowing cat-back exhaust.

Thought I'd share the solution.

I bought a Spectre PowerAdder hi-flow cone airfilter. It's adjustable to fit 3, 3.5 & 4" diam intake connections. (only $25, an oiled-cotton model with similar filtering/flow to K&Ns). Of course, the diameter of the airbox-to-MAF sensor connection on my '98 Rodeo is 3.25" - which is why I'm guessing you can't find a pre-designed Spectre, K&N or other hi-flow filter available to fit it - all the typical aftermarket stuff is typically either 3" or 4". (This might also explain why the CalMini kit is kinda pricey).

So I couldn't just bolt-on the filter, I needed an adapter. I was able to create an adapter from a length of 3" I.D. hard rubber connector tubing - I was able to stretch it to fit over the alum. MAF sensor inlet on the engine side, and on the filter end, it's outer diameter fit into the filter body with a nice tight fit. Tighten up the two worm clamps and it was in. I was able to position the filter inlet pointing right up to the stock fresh-air inlet by the driver's front fenderwall, and with the open-front face on this filter, it's gets a nice direct feed (and the whole filter sits above the inner fender, so it's not near any direct heat from the motor.) I still need to fab a better fender mount for it, and I'll likely add an alum. plate to block the back of the filter from the engine even more, but all in all it turned out pretty nice and relatively inexpensive.

Tom