The smallest that I've found in the domestic engines is the PH3614 size. There are probably smaller filters for the Koler and Onan engines and whatnot.
The poppet port on the Permacool sandwich has a 2psi breaking pressure. Unless you have some unreal volume, it shouldn't cause any issues.
Here's a 3614 size filter in WIX
Part Number: 51348
UPC Number: 765809513488
Principal Application: Various Chrysler/Jeep (82-06), Various GM (85-05), Lexus (90-06), Saab (67-06), Saturn (91-01), Suzuki (86-02), Toyota (88-06), Yugo (86-89), Harley-Davidson, Various Lawn&Garden, Farm Equip.
All Applications
Style: Spin-On Lube Filter
Service: Lube
Type: Full Flow
Media: Paper
Height: 3.404
Outer Diameter Top: 2.921
Outer Diameter Bottom: Closed
Thread Size: 3/4-16
By-Pass Valve Setting-PSI: 8-11
Anti-Drain Back Valve: Yes
Beta Ratio: 2/20=21/37
Burst Pressure-PSI: 275
Max Flow Rate: 7-9 GPM
Nominal Micro Rating: 19
Gasket Diameters
Number O.D. I.D. Thk.
Attached 2.734 2.430 0.226
It will be a little shorter in a Wall Mart ST3614. I'm sure that there are others.