Re: Fresnel lens for Ianabeam Blonde?

Ignoring the intense heat of an Ianabeam -- which will most assuredly crack the lens -- you will not be able to get the proper focal length needed for a lens of such a diameter that would be required. The optics would require that the lens would need to be further away from the lamp than the fixture will allow it to be mounted. You won't be able to get the full range of spot to flood -- one of the main attributes of a fresnel fixture.
Unless you're a real "do-it-yourselfer" who loves to tinker with arts and crafts projects I would suggest buying fresnels.
Open faced fixtures are designed a specific way for a specific purpose. If you're just looking to soften the beam use diffusion on the doors, or a chimera lightbank.

DS