Thorne, in addition to your original requirements you've further clarified that:
1.You do high milage trailering on rough roads.
2. You've already done the Harbor Freight / Northern tool cheapy trailer route at least twice, and you've extensively rebuilt these trailers and note that they are not for you.
3. You've owned an old style Holsclaw trailer which you liked, and which you feel provides suitable suspension.
4. You're considering building some sort of trailer if you can find a suitable design.
5. You own an EZ Loader galvanized trailer which you like, but you want additional shock absorbtion for a light boat.
Seems to me any commercial trailer, or trailer parts that you buy or build up, are going in be in the 1000 pound load category or heavier. Essentially you have said you need that sort of rolling stock for durability and reliability, and it's too expensive to get any sort of "quality" trailer with a "lighter" rating.
Seems to me that you probably recognize that building a "Holsclaw" style trailer from scratch involves a lot of additional details and work compared to a leaf spring style trailer.
Why not try taking your 1000 pound capacity EZ Loader trailer and adding foam planks to the bunks and see if this provides the cushioning you want for your lightest boat? The foam I was thinking about is Dow Ethafoam. You might have seen this in the past on some aluminum canoes, I forget the canoe brand, but they came with a big thick black plank of Ethafoam along the gunnels, and I recall that it was tough as heck and seemed to hold up very well under abuse. I'm thinking of the densest Ethafoam variety, Ethafoam 900 at 9.5 pounds per cubic foot:
Perhaps the lesser densities would work too, I'm just not sure of their durability. I've never tried to buy it, priced it, or looked for it, but it might be worth a try.
If this gives you improvements but is still not the cats's meow you can further experiment with altering the leaf springs on your EZ Loader per mcdenny's post.
Or, you might reflect further on how a modified bunk system or other support system can provide further cushioning on a standard trailer - without being specific and without actually doing the design, I have to think that the two big springs and three shock absorbers on the old Holsclaw trailer might switch on some mental light bulbs for a not too complex bunk suspension arrangement. Providing additional suspension at the bunks might appeal to you in that it would not alter or compromise the road worthiness of the conventional EZ Loader or other trailer. Or, require you to feel resposible for the road worthiness of an all up trailer design that you do from scratch