If your CO says "NO" to the mousepad/tape solution, as well as a simple stock swap to something akin to the Magpul PRS, then WTF precisely is he/she willing to permit?
I only ask because no matter what you do, it'll be "modifying" the rifle to one extent or another. If you really need more comb height to get setup on the rifle properly, then the best course of action is to lower the scope (whether by a new mount or rings) to achieve the proper cheekweld for you.
If that's a no-go, then the options are extremely limited and may ultimately need to be modified themselves to work properly. Those include "clamp-on" type cheek pieces (adjustable and otherwise) like the old "Delta" style cheek risers, those modified Deltas like the Masen design and those from B&C like their CVT offering which may or may not work on the M110 stock because of its LOP adjustment system. See the following links for more info:
John Masen Cheek Rest AR-15 A2 Buttstock Synthetic Black (may or may not give enough clearance for the longer, AR-10 CH to fully retract rearward)
Adjustable Cheek Piece for AR (may or may not work with the M110 LOP adjustment system)
Aside from those options, you have a variety of different strap-on options like the stock pack that cal50 mentioned. They come in any number of different configurations, styles, etc., but again...depending on how they mount up/attach and how far back you have get them mounted to clear the CH, you may or may not actually achieve better positioning on the rifle.
One last thought...and again...its going to involve drilling holes in/modding your M110 stock...is to source a Karsten style, two-bolt type kydex cheek riser assembly and installing that which will give you all the adjustability you could want, but may also need modded to permit the CH to clear/operate freely.