Yeah, I've had the same problem (a spring). I just made a tangible invisible FOF, instead of intangible. If the player has to be able to enter the FOF by the top, bottom, or sides, just give it the tag as such;
Intangible From Top
Intangible From Bottom
Intangible From Sides
Although they aren't listed in SRB2DBs tags (I think), you can find their tags for the specials reference document.