When I converted the 4 foot fixture using the Sunpark Instant start ballasts (not the same as the Sl15), I tested some 24" lamps by just putting the lamps into the loose end pieces. They worked fine. The ballast label shows a variety of available lamp sizes. I don't have the SL 15, so check the label to see if 24" are listed.
So, far from being an expert on this conversion, seems like 24" conversions would be handled the same way as 48" conversions.
Caveat emptor.

Have to add a comment.

Earlier in the Overdrive thread Zink described how he linked the pins of 2 x 24" tubes in series and then used a single ballast for the overdrive. Seems reasonable.

In another source on overdriving, the author did not make a distinction with 48 or 24" lamps.

At one point it was writen that one could overdrive a single lamp with a ballast meant for a 4 lamp throughput, ie a 4x OD, but there was a significant drop off in efficiency. This might be the same situation as for a single 24" lamp with the SL 15 ballast....a 4x OD(?)

If that actually is the case, then it makes sense to power 2 x 24" lamps in series with a single SL15 ballast. One would wire one side of the ballast wires to one end of the 'first' lamp, jumper the other end of the lamp (fixture)to one end of the 'second' lamp, and then wire the open end of the second lamp to the ballast. It looks just like the 48" modification except picture the jumper in the middle of the 48" lamp. Ya know, that sounds a lot more complicated than it really should.
Still probably works based on my simple experiment described earlier.
Good luck