I use a jig that is super low tech but has worked great for years. It consists of two pieces of angle iron, approximately 3in in length, one square piece of mild steel that's 1/4in thick and 4in x 4in, two bolts, four washers, two nuts, and two pieces of duct tape.
Basically it works by clamping the blade between the two pieces of angle iron. The tang is the part of the blade that is clamped. There are two bolts that run through the angle iron pieces and under the tang/handle of the blade being clamped. Then I set the two pieces of angle iron on top the 4in x 4in piece of flat mild steel. I surface ground the 4in x 4in piece of mild steel that acts as the base. I think it's more precise to have a one piece, flat base in addition to the pieces of angle iron, because it's easier to insure that it's level. I welded one of the pieces of angle iron to the top of the flat base. I tried using epoxy at one point and, even with gflex, it would come loose once you torqued it down. You can make it work with epoxy but it's ideal to weld it imo. It acts as the anchor point to the base for the blade and other piece of angle iron.
I use bolts around an inch or so long and just let the non-welded side hang loose until I clamp it on a blade. If you want, you can drop a small piece of steel, a little thinner than the width of the stock you are grinding, into the bottom of the gap between the two pieces of angle iron. That way they don't squeeze together under the blade. If you're using epoxy then you REALLY need to do this because that is what causes the anchor point to break from the base when it's torqued down.
Finally, I put two pieces of duct tape on the bottom of the base so it will slide well and not scratch up the tool rest.
As far as use goes, it's pretty straight forward. I like to put a small level against the blade that's clamped and make sure the blade is exactly level vertically. I've found that it works best for me to use the two angle iron pieces to insure the blade is level vertically and the base to insure that everything is level horizontally. I had played around with just using two pieces of angle iron, but then you have to make sure they're level both ways.
I set the bevel angle on the platen using a couple angle finders. I like to use two of them just to be sure. Sometimes, if I'm using the jig to do a larger blade, I'll clamp a longer sheet of steel to my tool rest. It needs to be level and solid though. The whole system depends on everything being level and solid. That means thick pieces of steel with no flex.
In regards to providing distal support for the knife, behind the bevel being ground, I use my hand. I don't use a ton of pressure and I rest the heel of that hand on the tool rest and slide it across it with everything else. It functions to provide the distal support and let me know if the blade is getting too hot when grinding after heat treatment. It's honestly one of the only variables in the system that isn't "measurable" so to speak. Kuraki has me second guessing myself though now talking about the flex, from not having distal support on the blade. Decades of reloading ammo, assessing variables, measuring tolerances, and have made me a little overly meticulous and paranoid lol.
I was thinking I could rig up a piece of steel to go along the back of the blade, opposite the side being ground, and support it while it's being pressed against the platen. Id like to avoid having to take the blade out of the vise, so I think maybe just use two pieces of steel around 1.5in wide by 1/8in thick or so. One could be 8in long and the other could be 4in. I could weld the 4in piece to the 8in piece so one side would be 1/4in thick. That way the 1/8in side could be clamped to the angle iron and the 1/4in side would extend off the jig and be thick enough to contact, and support the back of the blade. When done grinding one side of the blade the support piece could be taken off and clamped behind the other side. This description is kind of awkward so I'll try and take a picture here in a minute then f what m thinking.
Sorry for the wall of text gents, pictures would have probably sufficed. Morning caffeine sometimes results in long-winded rants with me lol.
this is only probe 
