Incra I Box problems.

@Stick486 has the way.

I haven't tried this with the I-Box jig, at least not yet, but you should be able to make a wedge and attach it to the sacrificial strip in the I-Box jig, to keep your work held at the correct angle during the cutting process. I have made a bunch of spare sacrificial strips for my I-Box jig and would not hesitate to modify one of them for this purpose. I used the original sacrificial strip as a template and cut about 16 more from a 2' X 4' piece of 1/4" MDF that I purchased at Home Depot for about $5. About an hour later I had made enough for my I-Box jig to likely last me the rest of my lifetime.

My "Kyrmi Original Mini Fence" jig (another form of box joint jig) came with wedge shaped fence pieces to use with the miter gauge on the table saw to allow doing this when making 5, 6, and 8 sided boxes. There isn't one ready made in the set for a 15 deg angle, but it should be possible to do this when using that jig as well.

An I-Box jig should also be capable of this if the needed wedge is made for it. You could either trim the work at the angle before cutting the box joints, or just cut the box pins longer than needed and trim the excess off after assembly. A little extra pin length on box joints, trimmed off after gluing and assembly, always produces the best result.

Please post photos of your attempt and success.

Charley

Use an angled backer

I made a simple angled backer for my shop-built box joint jig that worked well for making joints on an angle. The angle has to match what it needed for the angle of the sides, and you can see how well the joint look when assembled.