Has anyone with a 14-24 tried to modify a extension tube to hold gel filters...or know of any tubs that have that feature built in?

I've been trying to think of ways to put filters on the 14-24 and found a few solutions for the front. One solution on DPReview ws to make a filter attachment for the front but even so it would only work well on APS-C cameras. The other option would be to hand hold the filter but even 4x6 inch filter causes vignetting, and custom or larger sizes are really expensive.

The other solution would be to tape a gel filter to the rear element. But I wonder if we could mod an extension tube or tubes to hold a filter so that we don't have to tape anything to the rear element. and how would that affect IQ?