As you can see, I started cutting up the battery box and frame (wish I had something other than an angle grinder) in order to get the air filter to sit properly. I thought I had it close enough to the correct position the other day after bending up part of the cross piece, but the carb wasn't completely seated in the intake boot.

So first I cut off the part of the crosspiece I'd bent up. That didn't help. It was hitting the battery box. So I cut back part of the box. That's when I saw the filter might be hitting the bolts. Some button head bolts would gain me a wee bit more, but not enough. (There are short screws in the photo, because I got tired of removing and replacing the long ones.)

The filter was still angled like this.

So I've decided to scrap the filter and try this cone shaped one.
http://www.knfilters.com/search/product.aspx?Prod=RC-1250I was hoping K&N had a suitable size with an angled flange, but no.
Again, since I'm using a compact lithium battery, I'll have room for the end of the filter. If I need to, I'll move the rectifier elsewhere, like the inside back of the battery box.