Vendor at gun show said full kit would work on my DPMS LR308 but the Magpul buffer tube is shorter than std commercial tube and doesn't allow the 308 size BC to fully cycle. I ordered a new DPMS tube from Brownells which should fix it. Just wanted to give a heads up.
To add salt to my wound, I bgt a couple of CTR cheek risers only to discover they're not intended for AR style rifles (block charge handle). Same vendor said they would work great
Live and learn.
I believe LaRue makes a cheek riser that might work for you. It slides back when you need to charge, but since your putting it on a 308, I'm not sure if it slides far enough. Call LaRue to see if it works.
Not sure about your comment about fixing the problem by getting a new tube. The stock probably won't fully collapse due to a longer buffer tube. Not sure but that's what I'm thinking.
For my solution on my SR-25, I shorten a Spike's Tactical SS buffer and added high density tungsten powder to give it weight. Also had to shorten the buffer spring since it had too many coils which would cause it to bind.
Here is photos of me installing a UBR stock onto my SR-25: