You can use plastic oval conduit with T and E for wall drops, this is the much same as capping from a cable grouping and de-rating perspective, as the cables lie flat to the wall. In many cases it is much prefferred to capping from a maintanence consideration as it can be re-threaded and no risk of snagging. If the conduit wont simply warm up and then bend into a shape that reaches to let the cable into the back box holes, then you can use rubber adapter bushes, that are formed like a grommet at the backbox, and the tother end a tube that stretches over the oval conduit. These are a better solution really as they also stop plaster getting into the box. There is the same problem with capping too, that being flat, it doesn't really couple to the round holes in the back box all that neatly- mostly there is a bit of a void behind the capping just above the box, to allow the cable to flex into the round hole, which is then supposed to have a grommet, but often doesn't. Once plastered over, no-one is that bothered that the holes are not the same shape, so long as there is enough overlap of the holes to let the cable pass freely. If you are really bothered you could make the holes in the back box more oval, but I dont think many people would bother.
