Make your own jig out of wood. Cut the bottom piece with a face of 25* and a strip that bolts to it to hold the chisel. You don’t have to go back and forth with it, you can go side to side instead so that you aren’t dragging the jig over the sandpaper or stone. Lay a strip of HPL down and it will slide on it very nicely. Derek Willis posted a home made planer blade sharpening jig like that many moons ago.
That's got my vote, make your own. I know if I wanted one I would just make one; definitely for plane blade sharpening. But, don't need one, so no issue. Actually, I sharpen my lathe tools (when I had a lathe) and chisels on my small HF belt sander, been doing it for a lot of years. First started doing it after I read an article on one of the top custom furniture makers, and that is the way he sharpened his chisels. Figured if it's good enough for a guy who sells custom furniture for thousands of $, good enough for me. The article didn't say why he did it that way, except it was fast. My theory is that each time you use a chisel, it is at a slightly different angle from the cut before, and from the next cut - so it doesn't matter if the angle is not 100%, just so long as it is close enough. And that seems to be true. I do try to keep the same angle when I sharpen, but I don't really think it is a major deal. Different manufacturers, different angles, not a huge amount, but different. A plane blade tho, I would not sharpen without a jig, there the angle of the cut is always the same, so I believe it should be kept the same. Works for me.
Oh yeah, students would come in to that custom builder's shop and ask where he got the short chisels, because they were so nice to work with. He made them. He bought his chisels at his local hardware store, and by constantly sharpening them (he also used them as screwdrivers, prying the lid off of paint cans, etc., so they pretty much needed sharpening each time he used one), he shortened them, and eventually had to buy new chisels, because the old ones were too short. Really wish I could find that article again.