Hi all, looking for a little help…..

My new neighbors 60’s ranch had the garage converted into a family room.  Converted is putting it nicely, more like hacked, I digress.  Anyway, the hackers left the back side of the chimney exposed and painted the block.  The new folks would like it boxed in and I’d like to help but there is an AC return that would interfere with keeping minimum clearances on one side.  The return could be moved but before we go thru that exercise, is there an easier way? Is there any material that can be applied directly to the block that will be pintable and look ok?  Left to there own, they will probably PL some paperless drywall to it.  Quick and easy yes but it doesn’t sound code to me.  Any suggestions?

BB