Since you haven't done dovetails before, Jack, you might want to spend some time studying the geometry of the joint first, and then how that geometry is applied to the manufacturing of a jig. The additional wrinkle is that you said you want to determine the spacing yourself, which is a more complex task. Plus, the 28" size is wider than the 24" width of commercially-made jigs.
As you might guess, precision is the name of the game with dovetails. Not only do the male and female parts of the joint (pins and tails respectively) need to fit on one corner, the precision has to be such that the fit will follow around all four corners of the box. Any errors (lack of precision) will be additive as you move from corner to corner. So, a seemingly small error will become a big one during assembly.
For through dovetails, the tails are cut with the angled dovetail bit following a template with straight fingers, and the pins are cut with a straight bit following the template with angled fingers. The angle on the template must, of course, be the same as the angle on the dovetail bit being used.
If you look at the templates for the jigs designed to do through dovetails, you'll see that the templates typically have straight fingers on one side, and angled fingers on the other, all precisely aligned, so the two parts of the joint are cut by turning the template around. The versions of the machines that allow custom spacing (e.g. the Leigh DR4 Pro and the PC OmniJig) have movable individual fingers that are locked into a template base.
The instructions for the PC 4200-series (fixed-spacing) jigs include methods for attaching the template to a separate wooden bar that can then be clamped to the stock for making wider joints. Essentially, you clamp it in one place, cut those tails or pins, then reposition it on the stock, and cut the next set. Precisely repeatable alignment becomes the issue.
There are also "chest" jigs available for making wide panels with dovetail joinery. These, again, typically involve multiple repositioning of the jig along the end of each panel, introducing potential alignment errors with each repositioning.
If you have machine-shop capabilities, you could certainly duplicate the templates from one of the commercially-made fixed-spacing jigs to be long enough to avoid the repositioning problem. If so, use aluminum, not Masonite. The Masonite would wear too quickly to be practical. But, that doesn't accomplish your custom spacing.
The other alternative, of course, would be to lay out the spacing as you want, and then hand cut the tails and pins, as has been done for years. You'd likely need to do a fair amount of practice before tackling the chest, however.