Changing the angle ought to be straightforward enough. make the jig with a flat piece of MDF or plywood (as the original jig builder suggested) as a 'cutting deck' if I may call it that, but instead of fixing the edge at the pointy end, hinge it with two lengths of piano hinge, leaving a gap for the router cutter obviously. I would guess his jig's fixed angle was about 20 deg - you are not likely to be making a panel too far off that, maybe plus/minus 7 degrees or so? All you need do then is devise a solid clamping system to lock the cutting deck at your preferred angle - nuts and bolts, knobs, whatever you have.
I don't understand why you'd want to clamp it to the router table, since it will be pushing past the cutter. How you'll guide it consistently past the cutter will depend totally on your table and what sort of grooves, tracks or channels are in it - whatever is there will determine how you control the jig. Maybe...the original builder is just relying on the fence to keep it straight, notice that he has quite a long fence, and nothing seems to be running in the channels on the router table.
Here's his pic again for reference:
Have a go at it and refine as you learn - often I find the Mk II version of my jigs is when things begin to come right. Be careful not to have the router bit sticking up to an unsafe extent, or you'll end up with a custom bit to rout around corners like this one I invented:
On clamping the jig the OP is looking at... Because that one has legs/supports that would hit the bit if it moved. That jig is like a table extension, angling the table and having the bit through the table, so you slide the panel over the face of the jig as if it were the table..
From what you say, you seem to be used to the jig I'm used to which is an adjustable angle router jig (sled), which slides in the miter slot, the work extends over the lip of the sled over the bit at an angle and the work is clamped to the jig while it slides. You use piano hinge - I just use regular hinges.
Both jigs would do you. The jig you asked about would do you if you have a small table on your router table and your panels were long. IMHO- The one we mentioned would also work and might be more worthwhile to you in the long run. As being adjustable, if you find that the first angle you intended to use just doesn't look quite right to you (because it is too far or too close to the edge), you can change it, without having to build another fixed angle jig. And you can also re-use it for many other things.
Actually, that jig is just 2 pieces of rectangular stock hinged together on one side... 2 other pieces of stock cut long and narrow to drill a hole in one end (screwed to the bottom piece edge), other end with a slot that goes over a hanger bolt (and knob) in the upper piece to make it adjustable.
Tip on setting the angle? I just go over to my miter saw and cut 2 wedges of the angle I want, put it (temporarily) on each side, between the pieces of the jig and adjust to that. That way I know each side of it is even.
Good starting angle? 15 degrees on a panel jig is the most "often" used profile for creating panels, whether cabinets or doors... That creates a cut 75 degrees from the face -- 15 degrees from the edge/opposing side. How did I get that? << 90 degrees - 15 degrees = 75 degrees >>.
How long will that profile be??? Well since you have the angle (15 degrees) and your depth of cut... you could dig back to try to remember Geometry to use formulas in SOHCAHTOA and the Pythagorean Theorem... but personally, I just take a piece of scrap, cut the profile, see it, measure it and adjust from there. For me, that's a whole lot faster than me having to figure out the math.
Either jig, depending on the reach of your router, you might have to use a bit extension to get to the height you need to be. That is much safer that thinking you can just slide the bit out of the seat of the collet to get a little more reach... like it looks like John's example was. ::scary::
Alternately, this could also be made on a Table Saw. Using the same type of jigs (and a dado) --or-- a TS Raised Panel Jig --or-- by doing an asymmetric cove cut for the profile using a cove fence and then a dado cut to create the lip.