I just installed scope risers on my CZ 455 but now need to raise the height of the comb. Any advice on products or methods?

I am not sure if this applies as the only answer... but I shot a precision .22 match today and about 2/3 of the shooters had 455s or 452s and every one of them had some kind of aftermarket stock on them.

It's a fairly common issue with stocks not designed for optics unfortunately. If you do a web search for rifle cheek riser you'll see all kinds of options for sale or DIY. Essentially they involve putting some type of raised padding layer in the cheek weld area and some way to hold it down. The challenge is keeping it from moving around.
One of my rifles has a cheap black plastic stock on it. What I did was cut several strips of material from one of my wife's old yoga mats, roughly 5" by 2" and stack them until I found the preferred height. (I also was smart enough to cut the entire length off so it wasn't so noticeable.) I secured it with black self adhering sports wrap (grocery store pharmacy section) and it hasn't moved yet. Just to be a bit tricky I believe I also picked up a piece of black felt with self adhesive on the back at the local craft store so it doesn't chafe my cheek.

Or you can spend $30 for the same thing from Brownell's.

A simple method is to mold it yourself ...
I have done this MANY times ...
Simply buy some JB WATERWELD epoxy putty from HOME DEPOT ...
The whole tube of putty IF applied will increase your overall weight by 2 ounces ...
simply apply until comfortable , this will take TIME , but it will feel like an extended part of you're own body .

ENJOY ...
GSL .