I'm thinking for a 25 degree surface, just cut a block of wood to 25 degrees on a mitre-saw and then screw the plane blade flat to it with the blade end just proud.
Close, this is nearly all that's required but the details are different. One needs to be able to adjust honing angle for various applications (and sometimes on the fly) so it's best not to cut a honing guide to lock in a fixed angle.
Also, because fixing the plane iron or chisel firmly to the block is so important for consistency that aspect of the design needs to be implemented well, but the jig is still a very simple thing.
Note: honing angle is commonly 30°, not 25°. The latter is usually the primary bevel or grinding angle. You can however choose to hone at steeper or shallower angles for various tools and different applications (e.g. steeper for mortise chisels to take being pounded into wood, shallower for paring chisels which are only pushed by hand).
Ideally it would then have some sort of roller underneath so as not to have to do the work of sanding the block itself while sharpening - perhaps cut a rebate and insert a wooden roller?
A roller isn't necessary if the bottom edge or corner of the honing guide does not rub up against the sharpening media. If you are honing using abrasive papers or films this is fairly easy to ensure because the guide can run on the glass/granite/laminate surface the abrasive is stuck to but it takes a bit more thought if using diamond plates or any kind of stone.
Take all the above points together and you end up with a very simple, but brilliant, honing guide. Variations of this design, now made famous by Brent Beach, are given in this previous Answer that you might like to look at for more info, but I'll include the picture here again.
In this era where it's so common to think that throwing money at problems is the sure route to fixing them, I think it's worth highlighting again that the results possible with a jig like this (free or nearly so!) are just as good as those using any commercial honing guide, regardless of how much it costs.